It appears you created a auto-enter serial field for numbering your records. When you delete a record FM does not reset the serial number, FM just deletes that record. It does not appear that there is a problem. You can use get(RecordNumber) to get the record number and the record number would change when a record is deleted.
The record number will also change if you sort the records...be aware of this. Record Number is the number of record in the current found set, sorted by however you have it sorted at the time...
Depending on how you are using the number (hopefully not as a linking field to another table!) You can also reset the autoenter counter to the number you want to be on.
This may make sequential entry "sync"...but opens the risk of duplicating a number...
Of course this field should not be used in a relationship.
Resetting the counter would not help because you would still have holes in your count and possible have duplicate numbers. If you had 100 record and deleted records 6-10 (5 records) then reset your counter by subtracting 5 (back to 96) you would end up with 2 records numbered 96,97,98,99,100 when new records are added, with records 6-10 still missing. If the user delete more records before adding 5 new records then you could end up with 3 or 4 duplicate numbers.
I gave one option that might work for the OP. Enough details were not giving to state if would it work for them are not.
1. Does it really matter to have 'number jumps'? For many of us, the neat freak streak wants them, because it looks 'tidier', but actually they are just an abstraction anyway so does it really matter? (It's the actual description that matters.)
2. It's possible to script serial number updates but is this a level of intricacy that you'd like to go to? (You'll probably get lots of alternative strategies on how to accomplish this; FMP can do it in multiple ways.)