The 5320.20's good for up to one year. So if you think you may be returning to the same (or similar) location (in the same state) again, just define the period of the form for one year, as I've done in the sample I posted. Otherwise, I'm not aware to a limit on the number of form 5320.20s you can submit.
+1. And save the filled out PDF form so that you only have to change the dates from year to year. I just sent off three new 5320.20's on Friday. Same locations that I do every year, but these are for 2 new NFA items.