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