At the moment from what they're saying on the meteor-talk group, there aren't any enforced limits of any sort. Your app just sort of scales itself alongside all the others hosted there.
There is only one thing though, if your app isn't used/has no visits for a few consecutive hours its 'killed'. When someone visits it next time its put back up (of course the end user wont notice this, to them its as if it were up all along)
But what it means is background processes that you use/cron type tasks don't work well because the meteor deploy server will kill your app's task silently until the next web request comes along.