SQL Server Memory Determination
"Should I add more memory to the server" is a tough question to answer. Your box might be coping "just fine" with what it has, or you may find that tuning the system in other ways (expensive queries, for example) would be more effective. There are a number of resources that you should consider, including Jon Kehayias' How much memory does my SQL Server actually need, as suggested by Aaron in the comments.

Your system may have other bottlenecks apart from memory, particularly if there are a lot more databases, such as disk reads/writes. Writing to log files, reading from data files - it all adds up. But as far as memory is concerned, if you think your current system could cope with less RAM, then it should be able to cope with more databases and the same RAM.