In the AfterUpdate event of the table number field:
BidderNumber = TableNumber & Format(Nz(DMax("SeatNum","tblBidders","TableNum = " & Me.TableNum),0) +1,"00")
The Nz() takes care of adding the first bidder. The DMax() would return null and Nz() converts the null to zero. Then 1 is added to the maximum seat number found to generate the next seat number.
The Format() gives you a leading zero if necessary.
This does not account for gaps caused by moving people from table to table. You'll need more complicated code if you want to look for gaps.