The number of classes is only limited by disk space and the limit fixed by the site manager (maintenance / virtual classes). The default is 200.
The reasonable limit to choose depends on your machine for disk space and computation power.
disk space is not a problem on recent computers, you may allow 200 MB for each of 1000 classes on a 250 GB disk (need some space for backups).
The main point is the load average of the computer. It depends on the number of simultaneous users. A few hundreds of simultaneous participants is ok, say 20 classes of 20 participants. If each class works for 2 hours a week, equally distributed on the 5*8=40 hours of the week, that makes 400 working classes plus some ghost classes not using much power. You can begin with the default of 200 classes and 5000 total number of participants, then look at the load average (maintenance / current server activities) during some heavy hours, and adjust the limits accordingly.