The premise is flawed. I would not only count them properly in the first place, but continue to keep tabs whilst I supped, and adjust ratios as necessary in order to avoid unpaired leftovers.
I wouldn’t pre-count, but I cannot conceive of a world where I wouldn’t be adjusting the ratios for the last few slices once the discrepancy was obvious.
This is acceptable. It also indicates that you make a solid guess at the beginning.