Can a character have more than one Alarm spell active at a given time? There doesn't appear to be anything in the spell description to prevent it, and it's not a concentration spell. Is there some general principle that would restrict this?
As you said, there is nothing in the description to prevent it, and it's not concentration. The only limit to the number of alarms you can have is time. As it's a ritual spell, slots aren't even a concern for a wizard, so it's really a matter of how many 11 minute periods you have to play with.