When working with an identity column, a counter per table is maintained. This counter is used to generate new values for the identity column of the table.
When you load many data into a table as a test and then truncate the table, the identity counter is not reset. So if you inserted 1000000 rows, truncated the table and inserted an entry, you’d then the value 1000001 for the new entry.

If you want to reset the identity counter for a table in order to have the next value of the identity column be 1 instead, you can change the identity_burn_max attribute for the table e.g.:

sp_chgattribute my_table, 'identity_burn_max', 0, '0'

Please note that this command creates an exclusive table lock on the table. The lock is not kept for long but this means that the command will be blocked by any read or write lock on the table.

The last parameter is the identity counter you want to set. So if you have kept the entries with the identity values 1 to 10 and deleted the rest, you’d have to set the identiy_burn_max attribute to 10:

sp_chgattribute my_table, 'identity_burn_max', 0, '10'

If you try to set it to a lower value (i.e. a value lower than the maximum value already in use in the table), sp_chgattribute will fail, refusing to update the attribute because you then risk having duplicate values in there.

You can work around it by directly setting the attribute using dbcc:

dbcc set_identity_burn_max(my_database, my_table, 'my_new_value')

Also note that if what you want is to actually create an identity gap, all you have to do to increase the counter is to allow identity inserts on the table, insert a higher value and delete it:

SET IDENTITY_INSERT my_table ON
go
INSERT INTO my_table(my_identity_column,...) VALUES (1785,...)
go
SET IDENTITY_INSERT my_table OFF
go
DELETE FROM my_table WHERE my_identity_column=1785
go