I use scottrade. And it depends on the price of the option and how much confidence I have in it. I buy any where from 1 - 200 contracts at a time. Scottrade eats me up in commissions when i buy that much though
Scottrade user here not because they are the lowest commish but because I also trade in a host of other stocks in addition. I also have a "local" branch I can visit (and have) in person plus I've been with them for 12-15 years... I have a second account with Ameritrade (but no options with them). As far as contracts go, I buy in lots of 10 but don't usually exceed 30 in any given stock. That's just my preference.