The formula you should be using is this;
nPr, meaning n!/(n-r)!
n represents the amount of values, and r represents the amount of values that you actually take. Since in this case, you have 6 values, but you take all 6. The formula nPr is used when order matters and repetition is not allowed.
What do I mean by order mattering?
If order does not matter the combination [3,23,62] would be the same as [23,3,62]
However when order does matter the combination [3,23,62] would be different from [23,3,62]
So subbing this into the formula
Please note that ! in maths represents "Factorial" - this means you must multiply the number by every single positive integer under it, not including 0 since that technically isn't either positive nor negative e_o
= 720/0! (0! is equal to 1. This is the exception to the explanation I gave above about !)
Well, Jojo tested this on your calculator and I think we know the source of your problem now
There are two 65s, so the online calculator factors them as "duplicates"
>If you factor in types, allowing dual types, would that restrict the number?
aren't there 18 types...
commented 15 hours ago by Helios_Ex
If we were to take dual types, then the amount of possible eeveelutions that are possible is equivalent to the amount of possible dual types + amount of possible single types.
What is this?
6P2 + 18
= 153 + 18
So if we were to factor in types, allowing dual types, top of 171 eeveelutions.