Why an Invincible could operate only a small F-35 force

Could the Invincible class operate F-35s?

In theory yes, but only in limited numbers because the ship lacks the space of a larger carrier.

The F-35B's short-takeoff and vertical-landing capability solves the absence of catapults and arresting gear on an Invincible-class carrier. A ship of that size could therefore launch and recover the aircraft in principle. It does not gain the fuel, magazines, maintenance spaces, lifts, and deck parking of a larger carrier. The practical air group would remain small, limiting sortie generation and sustained combat value even if individual operations were possible.