I'm not sure there is a hard limit. This begs the question, what are you trying to do?
I'm working with a couple of projects that run scripts on the server. In my case the primary function is to setup data for a related iPad/mirrorsync file. One script runs every 10 minutes and updates a limited set of data. Another script runs once per day but can move/update 8000 - 10000 records. The second one runs in the dark of morning ( 1 AM ) so users are not affected.
I think there is no limit in disabled schedules, but it will be "uncontrollable" if so many schedules are listed in admin console.
Server help says "there are 2 queues ...", so if you define "messaging or script" schedule more than one, the schedules may run simultaneously.