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If you create a formula numeric type of field, you can reference the built in field Record ID # to increment your number automatically. If you're not familiar with creating new fields, please see our Add a New Field help topic.
Once you've created your formula numeric field, your formula would look something like this.
[Record ID #] + 100
This formula would take the Record ID # and add 100 to it for each new record. So if the Record ID # is 1, the formula numeric field would return 101.
If your sequential number needs to include letters, create a formula text field instead of a formula numeric field. Your formula might look something like this.
[Record ID #] + 100 & " " & "Support"
If the Record ID # is 1, the formula text field would return 101 Support.