Sequential Numbering in MS Access

I have a table with existing Job_Numbers.(17 Job numbers). Each month I will match this table with another new table via the serial number. For each new record, could be one or many, I want to apply a new sequential Job_Number. i.e. in this case start at Job_Number 18 then 19 etc. How can I do this? I am fairly new to Access so please be gentle! I hope this is clear. Many thanks.