This shows it too.
A quick check of the rosters shows that each team has between ten and twenty seniors, so let's say that each team has an average of fifteen seniors. That makes for a total of 12,650 players, with 1,725 seniors. But that doesn't count Division II, which has roughly the same number of teams, so double those numbers to 25,300 players and 3,450 seniors.
So the first lesson that our foray into math offers is that not every college football player makes it to his senior year, and being offered a scholarship out of high school is no guarantee of eventually entering the NFL draft.
So, including the fifty or so underclassmen who leave college and declare themselves eligible for the NFL draft, that's a pool of 3,500 players who could be drafted. Now consider the number of players who were drafted by NFL teams in 2011: 254. In other words, only approximately 7% of eligible players get drafted.
Those players then have to compete with everyone else on the roster, plus any undrafted college free agents, plus any other veterans or players from other leagues the team might want to check out, just to make it onto the Week 1 roster.