Maximum number of characters in a cell

If you are asking how many characters can you enter in a cell, the answer is a lot more than just 255, depending on your version of Excel. Allan Wyatt says 32,767 with some caveats  (https://excel.tips.net/T003163_Character_Limits_for_Cells.html). I frequently display more than 255 characters in a cell, though to do so usually means using Word Wrap and smaller font.

If you are asking how to determine the maximum number of characters in a cell from a range of cells, such as

A1 = abcd

A2 = abc

A3 = abcdedf

A4 = abcdedfghij

then you can use an array formula such as {=MAX(LEN(A1:A4))}

If you are asking how to restrict a cell to a maximum number of characters, then the answer is to use Data > Data Validation and choose "Text Length" from the Allow field.