This one depends all on you and how much you are comfortable to invest your first time investing in stocks.
The amount of shares you buy depends on the amount of cash you want to invest at the beginning.
Every company have different price for their shares which means that you have to suit what shares you can buy to the amount of money you are investing.
Lets say that you have $600 and you want to invest it all into one company which has their price on stocks at $10 then you are buying 60 shares in that company.
If the brokerage you choose doesnt allow fractional trading, you will have to round down. This means that if a share costs $61 and you have still $600 to invest then you will be able to buy only 59 shares of that stock. Becuse 60 shares would be $610.
By the way fractional trading means that you can invest as little money as an investment as you want to.