How much 120 from 240 is too much?
The number of circuits isn't really the issue. You could run as many as the panel will allow, number of breaker positions. The issue is amps to the panel. You've got 40 amps to use and as long as you don't exceed that for any length of time (you'd trip your 40 amp breaker)' you'll be fine. I'd say three would be fine Think of your house panel; you've got a 200 amp service but if you total up the breaker amps, you could get 450. This is OK because you don't run everything at the same time and the circuits aren't fully loaded.