According to prophet, 70,000 people from his ummah will enter jannah without reckoning, among them each will have 2 wives, what does it mean by 2 wives?? Does it mean that the 70,000 people will be only men not women?? What if a man has only one wife in this world then who will be the second wife??
Your first question:
what does it mean by 2 wives?? Does it mean that the 70,000 people will be only men not women??
No. Rather it means that the men in paradise will have (at least) two wives each. This is apparent from the version narrated by Abu Sa'eed Al-Khudri :
لكل رجل منهم زوجتان على كل زوجة
Each man among them shall have two wives
Also note that if you read it carefully, this does not just apply to the first batch of 70,000, but also to those who come afterwards - it applies to all the men in paradise. In another hadith it says that even the last person to enter paradise shall have two wives (See Muslim 188).
Also note that there are ahadith which mention more than two wives (such as for the martyrs) so two can be understood as the minimum number and some inhabitants might have more according to their status in paradise.
Your second question:
What if a man has only one wife in this world then who will be the second wife??
These wives are women who would be married to the person in paradise, not necessarily his wives from this world. If a person had no wives or only one wife in this world then this could be resolved in two ways:
He could be married to those human women in paradise who remained unmarried in this life. Or those whose husbands were disbelievers and hence could not enter paradise, leaving these women single. This interpretation is based on the assumption that both of the wives mentioned in this hadith are human (which has been adopted by some scholars).
He could be married to the Hoor al-Ayn, who are women made by Allah solely for the reward of paradise. The more likely interpretation of the hadith is that both of these wives will be Hoors and that they will be in addition to the human wives of the person. A version of the hadith states:
لكل امرئ زوجتان من الحور العين
Everyone will have two wives from the houris
The marriage in paradise to the Hoor al-Ayn is also mentioned in the Quran:
سندخلهم جنات تجري من تحتها الأنهار خالدين فيها أبدا لهم فيها أزواج مطهرة
We will admit them to gardens beneath which rivers flow, wherein they abide forever. For them therein are purified spouses.
وزوجناهم بحور عين
And We will marry them to fair women with large, [beautiful] eyes.
فيهن قاصرات الطرف لم يطمثهن إنس قبلهم ولا جان
In them are women limiting [their] glances, untouched before them by man or jinni -
إنا أنشأناهن إنشاء فجعلناهن أبكارا عربا أترابا
Indeed, We have produced the women of Paradise in a [new] creation. And made them virgins. Devoted [to their husbands] and of equal age.