How many pro tem moderators are chosen for a beta site? I know for Literature.SE there were 3, but does that always apply? Could there ever be 4, for instance?
The question about the initial number of moderators has been answered already. I'll add a bit about the number of pro-tems in general.
While three is the default number of moderators, you shouldn't hesitate to ask for an additional moderator if you need one. The number of moderators is determined by the demand, if you make your case why the current number is not sufficient, you can get additional moderators appointed. Three active mods is usually easily sufficient for new sites, but it can get a bit tricky if some mods are less active temporarily.
On Skeptics we asked for a fourth mod when two of the three original mods were busy with other stuff at the same time, and we got a fourth mod. There will always be real-life stuff that gets in the way of moderating, if you communicate that to the SE team you can ensure that they know about any bottlenecks and they can appoint another moderator.
We always start with selecting three.
If there is higher demand, we appoint more as needed. For example, Christianity has 5 pro tem moderators due to some amount of controversy and increased flag load when the site first went online.