Years ago, well before the Yankees employed two hundred broadcasters for one single game. each team had two broadcasters. There was the main play-by-play guy, and then there was his understudy. If a team was on multiple stations (say the New York Giants or the Yankees), then there might be four. Some teams had an analyst, especially once television became a part of the game, and as TV became even more crucial, the teams would expand to include more broadcasters.
However, the concept goes back to those days with one broadcaster and an understudy. The broadcaster would be the "voice" of the team, while the understudy would do primarily stats and research. Sometimes, the understudy would pitch in thoughts throughout the game - think Harry Doyle's assistant in "Major League". But usually, the second broadcaster would get a couple innings in the middle of the game. It was a way to break in and have some innings under your belt so you could get a #1 job somewhere. In the days before tapes and CDs made it easy to ship recordings off to prospective employers, you needed someone to take a chance on you and give you a couple innings per game for a year.
That tradition has held through to today, mostly for games that are radio only for that particular team. For example, the Pirates have five broadcasters - two play-by-play guys (Greg Brown and Tim Neverett, who took over from the retired Lanny Frattare) and three analysts (Bob Walk, Steve Blass, and John Wehner). Four work every game, with two usually on the television broadcast on FSN Pittsburgh and two on the radio side. However, if FSN doesn't have the game, the four will split time on the radio.
Basically, it depends on the contract and the status of each broadcaster. It's usually designed, though, to make sure each broadcaster does all 162 games (unless otherwise stipulated in the contract, like Blass, who does just home games. Or Vin Scully, who doesn't work east of the Rockies).
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