Billy is right here. You can have 85 players on scholarship at any one time. You can have 105 players in fall camp. You can have as many players as you want when school is in session, but note, however, that if you have 150 players on the football team, there must be an offsetting 150 women's players on various women's teams, per Title IX. Thus, the two real upside limits on the size of the team when school is in session are the number of qualified walkons, and the number of women in women's sports. Thus the Ohio team tends to cap out at about 110-120 players.

When you are on the road, there is a separate limit, established by the league for the number of players that can go to league games, which may be about 60, I'm not sure. The league may or may not establish road limits for OOC games, too, usually a bit higher, say 75.

Just to blow your minds on the whole number thing.  In my lifetime, it was routine for basketball teams to assign even numbers to home jerseys and odd numbers to road jerseys for the same guy.  For example, I'm 22 at home and 23 on the road.  Imagine the royalties if Jordan could sell jerseys numbered 22, 23, 6 and 7 (and 44 and 45?).  What a marketing opportunity lost.

hawaiian bobcat, I would guess that they normally dress about 110 players for home games. Of those, probably 15-20 are redshirting, though, so in theory you might be able to largely avoid duplicate numbers, however the numbers that are assigned may not work out that way. Also, regarding a place like Notre Dame, they may have a complicating factor - Some schools have a ill-guided practice of 'retiring the number' of things like Heisman Trophy winners, and never using it again. Some schools may have 5-10 numbers that have been retired, and thus have only ~90 numbers available.

You get a penalty for having two players on the field at the same time, so they try to assign numbers in a way that minimizes the chance of that happening, as in, they use one for an offensive player, and one for a defensive player, usually both reserve players. Current duplicates and unused numbers on the Ohio Roster:

1 - unused number
2 - Jamil Shaw CB (SR), Skyler Schwarzel WR (Freshman walkon, redshirting)
3 - Donte Foster WR (Sr), Leon Alexander S (Freshman, redshirting)
5 - Jarid Brown CB (Freshman, redshirting), Malik Rodriguez WR (Freshman)
7 - Shawn Harris CB (Freshman, redshirting), Daz Patterson RB (So)
9 - Kristoff S (Jr), Leffingwell QB (freshman, redshirting)
11 - Troy Hill TE (Sr), Cedric Brown WR (freshman, redshirting)
12 - Calvin Holloway S (FR), Jordan Reid Wr (Fr)
18 - Travis Carrie CB (Sr), J.D. Sprague QB (Fr)
20 - Ryan Boykin RB (Sr), Dyquan Stewart (Fr)
23 - Xavier Hughes S (Sr), Jake Johnson WR (Freshman, redshirting)
25 - Max Corcoran WR (Freshman, redshirting), Thad Ingol S (Jr)
28 - Dorian Brown RB (Freshman, redshirting), Randy Stites WR (So)
31 - Matt Amicone K (Fr), Tim Edmond RB (Jr)
39 - Cody Grillot LB (Fr redshirting), Jos Yazdani K (So)
42 - James Ray S (Jr), Blake Scipio DB (So)
46 - Matt Green K (Sr), Chad Moore LB (Freshman, redshirting)
48 - Zane Summers LB (Jr), Grant Venham P (So)
49 - Alex Asher TE (Sr), Davon Donovon LB (Fr)
54 - Tony Porter DT (Fr, redshirting), Lucas Powell C (So)
58 - Seth Everhart LG (So), Jerry WIlliams DL (Freshman, redshirting)
61 - unused
62 - unused
76 - unused
77 - unused
88 - usused

There are 20 duplicate pairs, and 6 unused numbers, which implies 113 numbers in use. It looks to me like there are enough linemen numbers to avoid re-using those, but not enough numbers for backs, so those get re-used a lot. They have tried to assign one to a freshmen who is redshirting, and one to an active player to reduce the numbers actually in duplicate use. Beyond that, they try to have one on offense, one on defense. Once you drop out the pairs that include someone redshirting, these are really the only potential conflicts:

2 - Calvin Holloway S (FR), Jordan Reid Wr (Fr)
Reid plays regularly on offense, Holloway is a reserve, and hasn't played yet. Even if he did, the chance of a conflict is very low

18 - Travis Carrie CB (Sr), J.D. Sprague QB (Fr)
Carrie is a starter, Sprague is the 3rd string QB. Both may play against Peay, but not at the same time.

20 - Ryan Boykin RB (Sr), Dyquan Stewart (Fr)
Both of these play. Since one is on offense, one on defense, it should be ok.

31 - Matt Amicone K (Fr), Tim Edmond RB (Jr)
Edmond has played on the placekicking team (remember, he caught a pass on a fake). If Amicone gets used as the kicker, there is a high chance of a problem here.

42 - James Ray S (Jr), Blake Scipio DB (So)
Ray did play some last year, and Scipio normally plays. With both of these being DBs, I can see a potential for a problem here. Since Scipio is out with an injury, it may not be a problem for now, but could be later.

48 - Zane Summers LB (Jr), Grant Venham P (So)
Summers has never played. This should not be a problem as he wouldn't be on the punting team, anyway.

49 - Alex Asher TE (Sr), Davon Donovon LB (Fr)
Neither have ever played. Also, one is offense, one is defense, so little chance of a problem.

An easy solution to this vexing problem: Allow three digits numbers.  Might have to change the font size just a little on the three digit numbers, but they could still be made quite readable.   The three digit numbers could be used primarily for third and fourth stringers.  Some might object to making the pecking order quite so easy to identify, but overall it would seem like a good idea to me.  

This is exactly why I think duplicate numbers are a problem.  This was not gamesmanship by CAL, it was a bush league trick.  Could you imagine if Ohio got burned on a play like this and if that play directly effected the outcome of the game.  People would be up in arms.

Yes, it's legal. Yes it's bush league.

Note that Ohio has a better option, a player that is both a QB and a punter, Derrius Vick. Marshall learned first hand that he can do both when he led a drive, then quick-kicked.

There are more freshmen than that redshirting. Those are just the ones with duplicate numbers. When there are duplicates, but one is redshirting, there is no chance of having both players on the field at the same time.