Usually if a contest says there are multipliers for "all 50 states" (and we have to assume this is US States? Without specifying a country or the actual contest we have to guess), that would mean that you would get one multiplier for each State worked (although what about DC? Presumably doesn't count as a mult?).
This would mean that the maximum number of multipliers would be 50 - one for each state worked.
Again - please specify more information. When saying "in a recent contest", it doesn't hurt to say which one it was :)
If you are asking how multipliers work generally, they are numbers that you collect as you are going. When you have worked out the score (you will get a number of points for contacts as specified in the rules of the contest), and the number of multipliers (again, specified in the rules of the contest), you multiply the numbers together to give you your final score.
Since you have now posted an example, I will quote from that PDF: "Multiply total valid contacts by the sum of the number of multipliers worked on each band."
That means that each 'valid contact' (defined in the PDF) is worth one point. The total number of contacts would then be multiplied by the number of multipliers on each band, where the allowed bands are "160, 80, 40, 20, 15, 10 meters only, except no 160 meters for the RTTY contest". The number of multipliers per band is given as 63 (50 states and 13 Canadian provinces). This gives a total number of 378 possible multipliers (6 bands x 63 states/provinces), and 315 for the RTTY contest.
So if you worked 10 stations on all 6 bands, but they were all in the same state/province, you would have 60 valid contacts and 6 multipliers, giving 360 points.
Of course, the cheat's way of doing all this is to use the EXCELLENT contest logging software by N1MM, where you enter each QSO as you work them, and it does the scoring for you.