Crash, the limit of background tasks depends. On what, I don't know. But I've had limit of 15 on my 928, but also it's been limited to 12 at some point. Not sure why the difference. I don't recall what it was when I had 822, but I've seen someone post somewhere that they were limited to 9. So apparently it has at least something to do with phone capabilities, but what, I don't know for sure.
As for how often they run, the documentation I've seen says something like, "Every 30 minutes, but can be adjusted 10 minutes either direction."
Christian, background tasks are allowed to run for up to 25 seconds (most are very short, probably less than a second in most cases). Since they can run up to every 20 minutes, depending on when the phone runs them (I'm using worst-case here), a single background agent can use up to 30 minutes of processing time per 24 hour day. Given that you can have up to 15 agents (depending on the limit, discussed above), you could be talking about up to 8 1/2 hours of processing time on background agents per day.
Now I've used worst-case numbers here to demonstrate the worst that can be. The fact is that most background agents don't take near that long, and like I said, probably most use less than a second of time, so you shouldn't be anywhere near this grotesquely extreme example. But it is something to keep in mind. If you install a new app and all of a sudden you're draining faster than you're used to, try disabling that new app's agent.