When you deleted some old records, their IDs are no longer in use. There is no necessity at all to fill in the gaps left by the deletion, neiter by inserting new records into those places nor by re-adjusting the IDs of other records.
If you want to get the number of existing records in a table, use COUNT:
SELECT COUNT(ID) AS [Number of records] FROM MYTABLE
OK, There is a trick to add identity values:
SET IDENTITY_INSERT MYTABLE ON
Thereafter, you can insert your IDs as integer values.
But that ought to be used carefully, under certain circumstances only: e.g. when migrating data from a different database.
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