The way the number of marriages is notated is a number after the "M" (standing for 'married') in the census sections about whether a person is single, married, widowed, or divorced. Here are some examples:
This one is listed as 'M1' indicating the person is on their first marriage.
'M2' tells you that the person is on their second marriage.
'M4' means that this person was on their 4th marriage.