JAR-FCL specifically says there is no limit to the number of ratings you can hold, however as Alexban says JAR-OPS restricts you to two. So if you're flying for an airline two is effectively the limit, but you can have more in private ops. At one stage I was current on five different types. Interestingly I used to find more problems moving between aircraft which were the same type rating but different variants (switches in different places etc) than between machines that were completely different.
Mind you I was a bit younger then and could remember things...
