If a man gives only one divorce to woman, can she marry someone else? Or she has to wait for 3 divorces before marrying someone else?
One divorce is still a divorce. Assuming she has gone through her iddah period, then yes, she is free to marry another.
The only difference between one divorce and three divorces is that three divorces is considered irrevokable; with one divorce, the couple still has the right to reconcile before the iddah has ended and retain their marriage.
For further details, check out my answer to the related question here: https://islam.stackexchange.com/a/113/22