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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, the following link explains how.
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.