Besides the ones already on the phone I'd say about 3 or 4 but me I only have 1 other then the defaults, but it all depends on how much memory you have free.
Posted from my CrackBerry at wapforums.crackberry.com
With an app like Aerize I believe themes may be archived on a media card. They must be on the phone to work.
Most themes are under 1meg. I often carry 10 or more on my 9000 without any trouble and lots less available memory than the 9700 has. So long as memory stays above 5 to 10 megs the phone should run fine.
Posted from my CrackBerry at wapforums.crackberry.com
I probably have between 10-15 themes on mine and haven't noticed any issues. Like another poster said, I usually keep the ones I really like and I like to change up my themes/walls pretty often.