How many alarms connect to fire lite MS-10UD-7?

just wondering how many alarms can connect to it, still kind of new to this stuff

Depends on the current draw of your signals you intend to connect to it. The -7 in the model number stands for 7 amps, so as long as the total current draw comes below 7 amps, you should be good.

And you also can hook up Simplex Devices to it because it also puts out Filtered DC instead of FWR.