Automations don’t understand formulas, so you would need to create a new formula field where the NEW NUMBER already exists. (In other words, you would create a formula field which is equal to your original number field +1.)
Then, set an Airtable Automation to set your original number field to the contents of the formula field, which will kick in whenever your trigger happens.
@ScottWorld - Must say - your posts are fantastically helpful! They have helped me many a time!!
I have just found this. Wanted to do the same, and the best I could think of was to add another field to my table (+ 1 of the original number) and insert that. Was hoping it could be done within the formula as it would be much cleaner but doesn’t seem to be possible. Oh well!