Ideally you want to purchase an amount that will keep your transaction costs low, no more than 2% of your transaction.
For example if you buy $100 of XYZ, your costs are 10% with a $10 commission. That cost goes down to 1% if you purchase $1000 of XYZ.

I personally try to buy in the old round lots, increments of 100 shares at a time. It is a widely used number making trades easier for the market makers and allowing my orders to fill easier on thinly traded equities, and I also use options contracts whih deal with 100 share lots. It is easier for me to keep track of everything going on that way.

Fool On!
Phil