That's kind of a loaded question, there. It's kind of like asking what shape will Jell-O be in when you nail it to a wall.
Ultimately, it's a tradeoff between station convenience and speed. More stations is more convenient to a point, but also makes for slower service. But, if people have to drive far to reach a station, they may opt to continue on in their cars and not bother with the train.
Historically, at least here in the mid-Atlantic, station locations largely evolved. A hundred, hundred and fifty years ago, stations tended to be closely spaced. As railroads grew, they added even more stations. Many of these original stations were also built at the request of local towns, landowners, politicians, or other influential people, and helped grow those communities. The close spacing was convenient for people since there weren't many alternatives. Trains tended to run somewhat more slowly, which lent itself to close spacing. People had to accept that it might take two hours to get make the 30-mile run from the farms of Bucks County into Center City Philadelphia, for example. As the automobile became more popular, railroads had to fight for customers, and pressure to trade off convenience versus speed increased. So, they abandoned or consolidated stations. Where they didn't abandon operations completely, that is.
The route itself also matters. Since your sig mentions the NEC, the NEC in the Philadelphia region is pretty random, and the station are generally further apart than on the other 12 commuter lines. It seems to be more or less that way from NJ up to NYC, too. The NEC is perceived as a somewhat higher-speed operation, even on the commuter side of things, and its connecting service to New York probably also plays a role in the current station spacing. But, getting back to Philly other commuter routes, those are more closely-spaced.
