After 75 moves, however, it's not optional, the game has ended. It's still a draw if the game subsequently "ends" in checkmate or a loss on time, though maybe not the players sign the score sheet, move on to the next round, etc.
And you can derive an easy upper bound from that as 50x8x8x2 (basically each 50 moves you make a pawn move)
if you only consider 3 moves repetition and not 50 move rule then this is harder and the number becomes one of those crazy combinatorical numbers.
This is not high enough, because the 50 move rule also resets when a piece is captured.
The 3 repetition rule is an opportunity for one of the players to declare a draw, but games can continue beyond that. The mandatory draw rule is 5 repetitions. In any case, the 50 move rule is far more limiting as to the number of moves in a game, since repetitions are necessarily neither pawn moves nor captures (the whole point of the 50 move rule being limited to those is that they are irreversible).
The 75 move rule is the exact same thing but mandatory. That has to be considered.
(same thing is true for 5 times repetition vs 3 times).
Captures also reset the counter, not only pawn moves.
BTW, the 3 repetition rule only comes into play is one of the players invokes it ... games can legally have more than 3 repetitions, but not more than 5 repetitions.
Compare this to, say, the L game, where the number of moves is unbounded.
If you read my comment that you responded to carefully, you will find that it is precise and accurate--as I said, the repetition rule has no bearing on the number of positions.
This horse is dead, so I'm moving on.
However there is a 75 move rule and a 5 time repetition rule that are both automatic (don't need to be claimed).