Su-15 were flown in flights of multiples generally of 4, operating in pairs.

In my personal first hand experience of seeing them react to our RC-135s and EP-3E’s in the Soviet Far East in the mid-1980s, I’ve never seen more than two Su-15s aloft at the same time. It was standard PVO practice to have two Su-15s on QRA at all times at each base (I’m familiar with Anadyr and Petropavlovsk-Kamchatskij), and in every case I ever saw, the pair were launched.