In this case, "pool" means a collective source. It's not a physical place.
At a guess, I'd say "a lap" can mean "a walk around for a look", which gives another wordplay: "cheesecake" can refer to a pretty girl, though this is fairly outdated. So a look around the available stenographers (a lap around the steno pool) will turn up only two pretty ones (two cheesecakes).
Disclaimer: I have never watched Mad Men.