I just ordered this from Amazon. How many mod spaces are possible?

The same as Oblivion and the same as the answer in your http://www.gamesas.com/index.php?/topic/1166292-beginnerneed-info-on-mods/: 254

Some people run into troubles with the game coughing up missing meshes much earlier though.

There is a hard limit of 254 active plugins that you can have running at any time. This can be circumvented to some extent by combining some plugins into larger ones.

But don't go out and start downloading everything in sight and installing it into your game - you're just asking for trouble if you do that. Start slow, read up on the mods you're interested in, and become familiar with programs like FOMM, BOSS and FO3Edit.

If you've never played the vanilla game, I'd suggest a vanilla playthrough first, so you can get some idea of what you want to change. It could narrow down the mods you want to try out by quite a bit.