Re: MMSI into Icom IC-M504
An MMSI stays with the vessel when sold, so if you entered one from BoatUS, it should have been the first. If it already had one, that number should have been transferred, rather than getting a new one. (I admit that the BoatUS MMSIs are a little "different"; I teach the ROC(M) course for CPS, and in Canada only Industry Canada issues MMSIs, which are indeed permanent to the vessel.)
So, chances are the MMSI already in the radio is its first, unless the radio was purchased used (given the number of non-DSC VHF radios out there, and how long sailors tend to keep equipment, I'd say it's unlikely) for that vessel.
That means that you should be OK for one more change, unless the boat/radio came with an MMSI which you replaced with another, then intend to change one more time.
What you could do is take the radio to an ICOM dealer who can factory-reset the MMSI (thereby zero-ing out the "only twice" counter) now, then program in your BoatUS MMSI. That will be the "first entry", leaving you with one more available for if/when you get an FCC MMSI.
Or you could get an FCC MMSI right now, do what it takes to program it in (via a dealer reset if necessary), and not worry about it again.
1998 Jeanneau SO45.2