The answer by Raffzhan to the Question, What did code on punch cards do with the other six bits per column? gives a good example of how information was expressed on punched cards, as does this article about computer cards for ICL (International Computers Limited) computers from Britain.
As others have stated in other answers to this your question, each column on a computer card represented one character, or one byte. Most cards that I am aware of contained 80 columns, so the limit for a single card was 80 bytes.
However, how much a stack of cards contained depended on what was stored on the cards and the format in which it was stored.
During the era of computer cards, the cards either contained computer programs or data, where each card represented on line of a program or one line of data.
A card with the program instruction,
C := C + 1
only contained six characters, so it only stored 6 bytes.
How much data was stored a single card depended on whether the data was integer numbers, floating point numbers, or characters and whether delimiters where used and whether spaces were used.
I have seen data cards where all 80 columns were used as if an 80 digit long integer was stored on them. It was a method of maximizing data storage. The program reading the data, usually a FORTRAN program would parse the data as required, so, for example, the first four digits may have been one number and the next six digits another number. This required numbers to have leading zeros to fill out the fixed format of the data storage. If the number read needed to be in floating point format is was converted by the program after having been read and parsed.