In my experience its not the number of open applications that matters, unless you're running high-CPU games and such. It has more to do with background syncing. Since the N9 has pretty good battery life, I have 3 mail accounts + twitter + facebook + gcal syncing all through the day. I suspect that when I schedule everything hourly they all happily start syncing at the same time instead of staggering it, so while that is happening in the background I'd see lagging animations, and when it's done I'd be back to smoothness - with the same number of apps running.

I usually only keep 3 apps open through the day (calendar, checklist, music player), but I've had short periods with 6-7 apps open with everything running comfortably.