In response to EdithBRTN's comment:

In response to moonslav59's comment:

In response to EdithBRTN's comment:

The average baseball career is 5.6 years. There are 30 teams with 25 per roster = 750 players. Divide 5.6 into 750 = 134. On average 134 players' careers end each year and the void is filled with 134 new players from the minors. 134 divided by 30 teams = 4.5 per team.

A quick stab at gussing how many call ups per year. This analysis is probably flawed but I gave it a try anyway.

Many of these "openings" are filled by prospects who never stick in MLB.

In the middle 1970s, the Red Sox were calling up about two players every year  who then stayed.

Very true. Their farm was amazing back then, and they had some aging players and poor bench players back then too. maybe it was easier to "move them along" and make way for the kids.