There is a little bit more to it than that, though. I agree that it is definitely capacity related, but it may also be relative to how much land is available at the airport, let alone time-long study in weather patterns at the area in question. For example, KVGT, KHND, and 61B aren't large airports compared to KLAS, but they all have parallel runways (12L/R and 30L/R at VGT, 17L/R and 35L/R at HND, and 9L/R and 27L/R at 61B). I can guarantee you that none of these satellite airports get as much traffic as KLAS, but because of how much they get, related to land available and wind/weather patterns, they have what they have.
By comparison, KSMF has parallel runways (16L/R and 34L/R), as does one of its satellite airports, KMHR (4L/R and 22L/R). But KSAC doesn't. It has 3 different runways altogether (2/20, 12/30, and 16/34). Mostly these are related to weather patterns and capacity (in MHR's case, it is a former AFB).
You can tell number of operations at a given field (on a given day) by looking at the Airport Operational stats on the airport's page at Airnav.
BL.