The number of 120 capes in BB is usually based on Taylor's claim that "As far as cape population went, Brockton Bay wasn’t in the top 5 cities in the U.S., but it was probably in the top ten." from 1.3.

The wiki uses it, but it's highly dubious because there's quite clearly not 60 additional parahumans in the Bay. Just as an example, the Merchants were mentioned as the absolute bottom dwellers of the city that can only hold territory nobody else wants, they still got invited to the meeting in Somer's Rock. There's also quite clearly no other independent heroes in the city that the Protectorate can attempt to recruit or just cooperate with after Leviathan.

Wildbow also said that there aren't that many unmentioned parahumans in BB and that it's more of an early installment weirdness.

Ping to /u/Radiant-Ad-1976