One option would be to use Row_Number()
with a subquery that returns the max()
:
Simplified solution:
insert into ListDetail
select 12345, sq+row_number() over (order by (select null))
from company, (select max(sequence) sq from listdetail) t
INSERT INTO ListDetail(ListID, Sequence, CompName, CompAddress, Date)
SELECT
12345,
sq+row_number() over (order by (select null)),
Name,
Address,
GETDATE()
FROM Company, (select max(sequence) sq from listdetail) t
WHERE CompanyType = 3