How would I make a table of autonumbers/sequences?

I want to make a field in a table contain numbers that increment each
time a record is updated.  How would I go about doing this?


Example:

Table1
CustID     balance   
A12        255.32
B20        132.20

Table2
CustID     transactions   
A12        7
B20        33


Every time something is inserted or updated in table1 I want to
increment that customer's number of transactions in table2.  How would
I go about doing this?


TIA, 
Joseph