insert into partner(
cus_no,
cus_name,
TP_id
)
select 
cus_no,
cusname,
123001+1????,  
from acecustomer


Data would look like:
ABC  customer1    1230001
DEF  Customer2    1230002
GHI  Customer3    1230003

I need the TPID to start at a specific value. 

DECLARE @StartValue int

SET @StartValue = 123001

insert into partner(
cus_no,
cus_name,
TP_id
)
select 
cus_no,
cusname,
@StartValue + ROW_NUMBER() OVER (ORDER BY (SELECT 1))
from acecustomer