Strictly speculation but:

Sonny tells Tom: "One hundred button men on the streets...if Solozzo shows a hair on his ass, he's dead." I assumed he meant almost the family's full strength in addition to the 50 at the Mall (assuming he kept all 50 at the Mall after he found out that Paulie was the traitor).

As for recruiting new people: Perhaps Michael and Vito let their numbers go down after Vito retired--the better to make the other families think they were slippin'. And maybe the new "secret" regime was meant to bring back strength for the Great Massacre.

All in all, I will GUESS that the Corleones didn't have more than 200 - 250 made men.