I was told recently by a mufti that imam ahmad ibn hanbal memorised 1000000 saheeh hadeeths. Now i dont know of any sources where there are 1000000 saheeh hadeeths. Bukhari apparently has approximatly 2000 saheeh hadeeths. So how much knowledge still exist percentage wise if possible or any answer sheding light on the matter would be accepted.
You should know that early scholars used to call or count each chain of a hadith (with any difference in the chain) as one hadith. This means these scholars used to count repetitions of the same matn (content).
So when al-Bukhari says he has memorized 600.000 ahadith (I've never heard somebody pretending he memorized an amount which goes beyond this) then in fact these were 600.000 different chains of many ahadith you could easily find him memorizing 10 or more chains with a hadith (note that earlier generations also considered saying of Sahaba as such, but al-Bukhari in his sahih aimed to chose only connected ahadith which are sayings or doings of the Prophet()) of the same content and he has compiled in his sahih and other books most of these ahadith in many occasions with different chains. Imam Malik has chosen for his Muwatta' from a pool of 100.000 ahadith covering fiqh rulings in first place, I remember that he said he left out 2000 ahadith he memorized from his teacher ibn Shihab az-Zuhri. Imam a-Shafi'i when asked about the amount of ahadith on fiqh he said: They are 500, when asked about how much of that is in al-Muwatta' he said all expect 35. Actually al-Bukhari and Muslim toke from the 700 and some ahadith (around 500 without repetition) which can be found in the different narrations of al-Muwatta almost all and left out 50.
There might certainly be some ahadith that have not reached us due to the fact that some libraries have been demolished etc. or people died without transmitting them further. But it is hard to give a rate or clear amount of that, even if this might be only very few (if we concentrate on sound and sahih narrations) as I'll try to explain.
As matter of fact the two sahihs contain around 2980 ahadith (without repetitions) which can be considered as sound, scholars such as imam Ahmad and abu Dawood and other scholars came to the conclusion that there are 4400 sahih hadith (sound and connected narrator chain and sound content, note that earlier scholars considered what modern scholars define as hadith Hassan as part of the da'if unlike sheikh al-Albani did in his as-Silsila as-Sahihah) if you toke sunan abi Dawod then you may find 2500 more ahadith (with a connected narrator chain), if we added sunan an-Nasa-i and Jami' at-Tirmdihi to that we would reach ana amount of 6200 hadiths which are not in both sahihs, in ibn Majahs sunan there are 600 more and in al-Muwatta' 50 beside this you may add known ahadith from nayl al-Awtar نيل الأوطار (Which covers all fiqh related ahadith only) and Musnad Ahmad which are not yet counted and reach a sum of around 11830 hadith which are compiled in books we have now in hand most of these ahadith don't reach the level of sahih, but there's a good chance that they cover almost all what the Prophet () said or did.
Note that we so far didn't councountless other sources as the 3 encyclopaedias of imam at-Tabarani (al-Majma' as-Saghi, al-Awsat, al-Kabir) nor sunan ad-Darimi nor al-Hakim's al-Mustadrak nor the books of Abdurrazak, ibn Abi Shabyah, al-Mustadrak the munsads of a-Shafi'i, abu Hanifa etc. (source of my statistics)
The scholarly opinion is that no tradition of the Prophet ﷺ was lost, albeit that some or a lot of hadiths were not documented. The opinion is, through repetitions or similar events, every required tradition was covered.
First, the reference to Imam Ahmad ibn Hanbal memorizing 1,000,000 hadiths was mentioned in several books, including his own Musnad:
وأول طلبه الحديث سنة تسع وسبعين، وله ست عشر سنة. رحمة الله تعالى. قال عبد الله بن أحمد: سمعت أبا زرعة يقول: كان أبوك يحفظ ألف ألف حديث.
NOTE. My own translation, so treat with care.
..., and he [Ibn Hanbal] started to seek hadith in 79 [A.H.] when he was 16 years of age, may Allah have mercy on him. 'Abdullah ibn Ahmad [ibn Hanbal] said: "I heard Abu Zar'a say: 'Your father had memorized a thousand-thousand [one million] hadith.'"
In Siyar A'lām an-Nubalā' (12/187), Adh-Dhahabi said that this was an accurate account of Ibn Hanbal's knowledge of hadith, and that during his time, hadiths with the same matn (content, Arabic: متن), athars, fatwas of the tābi'is, and tafsir were considered among what people counted. Then Adh-Dhahabi continued to say that the hadiths with unique content are less than one-tenth of what Al-Bukhari had memorized.
Second, it is worth noting that the number of hadiths you are attributing to Al-Bukhari (7397 total hadiths, 2602 hadiths without repetitions, or 2761 hadiths if you include his additional 159 mu'allaqāt) are only what Al-Bukhari included in his book Al-Jāmi' as-Sahīh al-Musnad al-Mukhtasar min Umūr Rassūl Allah wa Sunanih wa Ayyāmih (also known as Sahih al-Bukhāri) based on some specific conditions. - Not all hadiths that Al-Bukhari considered authentic were compiled in his Sahih. Al-Bukhari authored several books other than his Sahih (e.g., Al-Adab al-Mufrad, At-Tarīkh al-Awsat, At-Tarīkh al-Kabīr, etc.) where he included other authentic hadiths. Al-Bukhari himself said that he memorized a 100,000 authentic hadiths and 200,000 non-authentic hadiths (see Kitab al-'Amal as-Sālih 2/6).
Finally, as for how much knowledge in percentage still exists, this is a question to which its answers seeks to quantify how much we know of what we do not know. We do not know how much was lost of the hadiths, or of the traditions and the teachings of the Prophet ﷺ that never got documented in the first place. However, there is no known disagreement among scholars that the Sunnah of the Prophet ﷺ is a revelation from Allah ﷻ (see Islam Q&A's article "The saheeh Sunnah is wahy (Revelation) from Allaah"), and that Allah ﷻ has, is, and will always guard it (see Qur'an 15:9). Therefore, the scholarly opinion is that there is no tradition of the Prophet ﷺ that is meant to be part of Islam that was lost, otherwise, the perfection of Islam and the completeness of Allah's favor on us as stipulated in Qur'an 5:3 are not valid.
Practically, we are only accountable for the traditions that reached us. We are not accountable for what we do not know. For a brief account of how hadiths got documented, refer to this answer.