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Is is possible that you are using an autonumber field as part of your numbering? If so, it is not possible to go back and have the autonumber field fill in previous values that were deleted. There are some work arounds that involve using a mix of autonumber and formula fields. If you don’t need to keep existing numbers, it is also possible to redo autonumbering, throwing out all existing numbers and starting over. Often neither of the workarounds fit, though.
Another possibility is to reconsider whether or not it is really necessary to include the missing numbers. The missing numbers can feel wrong from a spreadsheet point of view, but often a database can function just fine without the missing numbers.