you cant put a number on the amount of shares yo have to buy to make money...
if a company has only 100 shares in it, then buying 50 shares gives you half of the company....
i the company has 200 shares, and you buy 50 you only own a quarter....
therefore you have to judge the price of the share, against the number of shares available (which values the company) and try an predict the share price going up or down.....
really you should just look at it as how much money you put in plus the percentage your shares grow....
over the long term invest in a big established company microsoft, apple, bskyb etc......in teh short term, only your guess is as good as mine