as usual in IT the answer is 'it depends'. How many portals you're running doesn't really matter, it's how many IIS instances that count. If you're running a single instance in IIS, but using multiple portal alias's, then you only have one 'real' website - under windows 2003 this is typically in an app pool. The first request (for any site under that instance) will load the dlls into the app pool, they're not loaded per portal. An app pool recycle can happen for a number of reasons, such as a scheduled recycle, an unstable pool or a pool which has been shown to be inactive - often this means that after 20 minutes your app pool will be unloaded. However, you could have a setup where you have seperate IIS instances per portal, but each of them point to the same physical file structure, in which case each of these sites could expire independantly - it's impossible to say without seeing your setup.
If you have only 1 IIS instance, then one portal will use the same amount of space (in terms of modules) as 10 portals. However, as ram is also used for items such as cached pages, permissions etc., each portal will also consume additional space.
If each individual IIS site has it's own app pool then you will automatically use 30MB per pool, plus you will load a copy of all the dll's into each app pool.
Note: app pool recycling is designed to help ensure that sites run smoothly, running keepalives can ensure that sites never unload, and you may find that hosts force app unloads automatically for this reason (e.g. i have read reports that certain hosts have regular recycled app pool recycles e.g. every 30 minutes, to try and maximise the number of sites per server)
Cathal