Even if you want to strictly separate them, the Robins still individually got more alts each than Marth ever has. In general it is seen that the Robins are so interchangeable (as are the morgans and Corrins) that past the base defaults, they shouldn't get more alts combined than other individuals. Grima is a reasonable counter to the issue since he is an entirely different persona, but it reaches a point of ridiculous when we've seen some version of Robin 6 times now, with the last 2 having been extremely close to one another, when so many units aren't even in the game and so many haven't have any seasonals or alternate versions yet. If you want to give each Grima their own single seasonal too, we could be looking at 8 Robins...

Hilariously you already can do a full Robin Emblem team + Future Past team, with MRobin, FRobin, Summer Robin and Christmas Robin on one team, and MGrima, FGrima, MMorgan and FMorgan on the other tea,.

Marth, well known as the face of the franchise, has yet to receive a single alt or seasonal. He's been in multiple games, so you could probably argue he deserves each game's version at least, and even his popularity doesn't seem to convince them to cut him some slack.