you can set your field as auto-incremented, in this case filemaker will assign a number to every new record based on that incremented value.
Is it possible to have the number generated be random and not based off of an increment?
you could use a calculation field with "set variable ( round ( random * X ; 0 ) )"
with X = (10, 100,1000,.... based on how many numbers you want )
but in this case the calculated field may not be unique ( depends if "random" give the same number twice, who knows !! ),
so in my opinion the auto-incremented value would be the same solution.
What problem are you trying to solve by using a Random number instead of a serial number of this?
It could be possible to generate a table of unique random numbers. From this table, you can select a random number and then delete that record so that you are selecting a new random number with each new record in your main table (or a serial number field in your main table can link to a serial number field in the random number table in order to connect to a different record...)
Either way, you'd need to generate new random numbers with each time you use the last record in this table and you can encounter problems in a network environment when you have two users attempting to do this at the same time.