How many applications can I run simultaneously on a new Macbook Air? With two user accounts running simultaneously with fast user switching, is there any issue in running say 10 apps simultaneously in each account? 

I have a personal setup and one I use for when I'm at work so that I can create two full workspaces that have different accounts/preferences, etc. But I like to be able to fast user switch between the two. In each account I have Mail, iChat, Reeder, Safari, Skype, iCal, iTunes, iWork open all at the same time. I used to do that on my old Macbook, but I would only have one account logged in at a time. Now with my new Macbook Air, it seems so far that it's ok handling all of those apps running at the same time in both accounts. 

Can someone explain to me exactly how it's managing all of that memory and resources and what performance (battery?) issues I might encounter. Is there a generally recommended limit to the number of simultaneous processes for a machine with 4GB of RAM, when those processes are pretty basic?
There are no system resources that can meaningful be counted in "applications", but some programs hog system resources atrociously, while others use hardly any.

If your computer seems 'burdened', then run activity monitor to learn what processes are using the most CPU and RAM, especially processes you aren't actively using.  You should avoid leaving badly behaved processes running continuously.