Two. One to take notes while the other screws it into the faucet.
or
The question is irrelevant, since you couldn't find the dolls even if you knew how many. (Note: Well, this was a good joke in 1983-84. . . .)
or
I got you covered.
or
Two… one to tell the engineer to do it, the other to say "I don't know, what do you think?"
or