You can use the row_number() function for this.
INSERT INTO PM_Ingrediants_Arrangements_Temp(AdminID, ArrangementID, IngrediantID, Sequence)
SELECT @AdminID, @ArrangementID, PM_Ingrediants.ID,
row_number() over (order by (select NULL))
FROM PM_Ingrediants
WHERE PM_Ingrediants.ID IN (SELECT ID FROM GetIDsTableFromIDsList(@IngrediantsIDs)
)
If you want to start with the maximum already in the table then do:
INSERT INTO PM_Ingrediants_Arrangements_Temp(AdminID, ArrangementID, IngrediantID, Sequence)
SELECT @AdminID, @ArrangementID, PM_Ingrediants.ID,
coalesce(const.maxs, 0) + row_number() over (order by (select NULL))
FROM PM_Ingrediants cross join
(select max(sequence) as maxs from PM_Ingrediants_Arrangement_Temp) const
WHERE PM_Ingrediants.ID IN (SELECT ID FROM GetIDsTableFromIDsList(@IngrediantsIDs)
)
Finally, you can just make the sequence column an auto-incrementing identity column. This saves the need to increment it each time:
create table PM_Ingrediants_Arrangement_Temp ( . . .
sequence int identity(1, 1) -- and might consider making this a primary key too
. . .
)