I'm not a SNG expert and opinions may vary, but when I used to play them regularly, one rule of thumb I had was to use the 5% stop loss rule as a guide. You don't want to lose more than 5% of your bankroll in one day, so I calculated the average number of SNG's I wanted to play in one day and arrived at an amount that complemented that. So, for example, if you had a $100 bankroll, the most you could lose in one day is $5, so you could technically play a $5 SNG, but if you lost it, you'd be done playing poker and that's not a good way to build a roll or get experience, so obviously, you'd want to do $1 SNG's or less. As a general rule of thumb, you could say 50 buy-ins for SNG's and 100 buy-ins for MTT's. There are more variables to consider like win-rate, ITM%, ROI, and the like, but I think the main thing is that you give yourself room to play several tournaments per day while not losing more than 5% of your bankroll per day if you brick out in all of them.

WPP = Average Number of Word's per Post