Dear,
There is no real limit, you can run the deployments to over 3000 computers. There are a number of deployments that can run simultaneously. You can set this under Configuration\Server Options, almost at the bottom under the Service Options.
All other deployments will be added to a queue and will be executed when a thread becomes available.
If you are running that many packages at the same time, have a look also at the number of simultaneous users on the DefaultPackageshare, this is also limited to 25 by default.