How often can you buy & sell the same stock?

As often as you have funds to cover the fees.

While you will lose money, if you are paying fees, but as often as you want.

It depends upon the market value, if it seems that market will down then I sell the shares and when down I buy the shares, this method helps me to make more money. For Same stock I am not sure how many times I buy this stock. But I always like to buy quality stock.