How can Stockfish rate a position to be +50 or greater?

After move 46 on the following puzzle: lichess.org/training/QB67i. Stockfish 11 gives white a +51.1 score. I'm curious where this number comes from, as there are not enough pieces on the board (I have a single pawn) to add up to 51 points. Is "mate" assigned some finite value that comes into play here?

Yes. I don't know how much it is with stockfish but "Chess engines usually assign the king an arbitrary large value such as 200 points or more to indicate that the inevitable loss of the king due to checkmate trumps all other considerations"

en.wikipedia.org/wiki/Chess_piece_relative_value

I think with those huge values it's calculating all the material from promoted pawns, but it's still difficult for it to find a forced mate at that depth when there's a lot of untraded pieces.

The score has nothing to do with the material count. The score has been adjusted, so that you as a human better can understand it. A score of +1 means that white is "better" by being up one pawn or better in the position - more space, more active pieces, .... When stockfish traverses the game tree it can evaluate positions and "see" who is better in the current position. Mate is not a value, - stockfish will show it as e.g. #-8 which means black wins with checkmate in 8 moves, if both sides plays perfect.
A score of +51.1 just means that stockfish is very very very very sure that white will win, if both sides play perfect, but at the current search depth with the current pruning of the search tree, stockfish has not really seen the mate. Let it search deeper and you will see.

Exactly, I like to think of it as:

+51.1 = white is totally winning
#[x] = white can force mate, no matter what

#4 Your explanation just seems very confused to me. You assert that the score has nothing to with the material, but then immediately explain how it relates to the material. Then, when it hits 51.1, you characterize it differently and suddenly the number represents some nebulous concept of how certain victory is. Why not just represent it as a percentage at that point? Certainly that would be cleaner if that's what the score was trying to convey.

But no, it's just a calculation of relative material, with the value of material fluctuating based on criteria that has been defined for it.

White or Black ???

@eVeLpIs +x means, white is better, -x means black is better.

@Feuer1 thanks

Now I understand !!!