It seems to me very difficult to make more an average of 50 back links per day. Is it ok to buy back links from some websites? Will google block my adsense account if I buy backlinks for a site which has adsense on it?
You can buy if you know how to buy and what kind of links to buy. Focus on relevancy and low OBL. About banning, why should google ban you because you have links on websites using their adsense service - so no worry about that. p.s. dont go just for numbers... 50 per day is a lot if you have new or weak website and will be risky!
Its against Google Webmaster guidelines to buy links. If everyone could achieve good rankings by doing this Google SERPs would become meaningless