You can use subtraction to figure out how many numbers there are from one endpoint to another. For example, in the set {1,2,3,4,5} we see 5 values. Subtract the large endpoint from the smaller and we get 5-1 = 4, then add 1 back on to get 4+1 = 5. The formula is a-b+1 where a > b.
Here's another example: {8,9,10}. Clearly we see 3 values. Using the subtraction rule mentioned above, we get a-b+1 = 10-8+1 = 2+1 = 3. This method is preferred if you have a tons of values you don't want to count out one at a time.
Such as this example {580, 581, ...., 993, 994}. We have a-b+1 = 994-580+1 = 415 meaning there are 415 items in the set {580, 581, ..., 993, 994}