Do bear in mind that even the absolute largest city in the SCSverse (which for some reason is Albuquerque) doesn't have this many depots. In fact, aside from Albuquerque, the cap is generally 12, and even that's usually reserved for megacities like Madrid, Saint Petersburg, Los Angeles, and... apparently Redding, because someone on the rework team had an extra coffee that day.
But in any case, the only way Billings (which also has Laurel right next to it, for extra depots) is getting anywhere close to that many depots is if basically every city in ETS2 and ATS gains about 50-60% more depots than they currently have. And indeed, there's a perfectly valid and reasonable argument that they should... but that's not a "Billings rework" or even a "Montana rework", but a "two entire honking games" rework, so it probably isn't happening anytime soon. Especially when some of the east coast cities will probably be struggling to even fit this many.