I count 41 on the 40-man roster. That is, I've included free agent lefty reliever Jake McGee despite no official word from the Giants about the signing.
There are 20 pitchers on the Giants 40-man roster (according to the website): Caleb Baragar, Tyler Beede, John Brebbia, Kevin Castro, Johnny Cueto, Anthony DeSclafani, Camilo Duval, Jarlin Garcia, Kevin Gausman, Trevor Gott, Conner Menez, Reyes Moronta, Dedniel Nunez, Wandy Peralta, Tyler Rogers, Gregory Santos, Sam Selman, Logan Webb, Matt Wisler, Alex Wood. The number goes to 21 if you add FNG McGee. Someone will get bumped from this list once they complete that deal.