How many people are recorded in a laugh track?

There is no set number, but there are various ways to get a laugh track.

On TV sets for the last 20 years or more, the "laugh track" is actually a live studio audience (of various numbers - 20 people on a small production, up to over a hundred for a show like Cheers or Seinfeld). In those cases, they film the episode live over the course of an hour or two, and they either have an "applause" sign that lights up when they want the audience to laugh, or a production assistant is standing there with the script, and signals to the crowd. They all laugh, then quiet down when the sign goes off. All of this is explained to them before the show, of course, and trouble-makers can easily get kicked out of the taping.

There are also laugh tracks provided by studios, for royalty free sound effects collections and such. In those cases, the laugh track can be recorded by a dozen or more people, or even as little as 4.

I was once hired with three other people to say one name in a chant, with several variations. They took our recordings and produced a stadium effect, as if thousands upon thousands were chanting.