This may be a dumb quextion, but why do you to store the next possible race number in a seperate table?
Wouldn't it be much easier to create a Query that tells you what the next race number should be?

The following SQl pasted into a Query in Access should give you this answer, without having to store the value in a seeperate table.
Code:
SELECT Max([tblNumberGen]![RaceNumber])+1 AS NextRace
FROM tblNumberGen