Yes, he has to summon the number that he declares.
Arachnids are the only really relevant thing that Ramos can summon in the most part, and at the cost of an 11+ of tomes, a scrap marker, and a soulstone every turn to summon three spiders, that's fairly taxing on a players resources. Other factions have the ability to summon significant models as well, and they are all slow for the turn they are summoned on top of being insignificant. Arachnids are also effectively either tarpits or scheme runners. They do not have a great damage track, or melee attack (which is offset by "Latch On", but I digress). They're fine as is.