Top N distinct

What does "recent 35 customers" really mean? If it means the last 35 purchase orders, which could be fewer than 35 different customers, use this query:

select columns

  from POHeaderDetails H
inner
  join POPartDetails P
    on H.POnumber = P.POnumber
 where H.PODate in
       ( select PODate
           from POHeaderDetails H2
          where 35
              > ( select count(*)
                    from POHeaderDetails
                   where PODate > H2.PODate )
       )

However, if you want the last 35 customers who purchased anything, and the last order for each one, then use this query:

select columns
  from POHeaderDetails H
inner
  join POPartDetails P
    on H.POnumber = P.POnumber
 where H.Customer in
       ( select Customer
           from ( select Customer
                       , max(PODate) as maxPODate
                   from POHeaderDetails
                  group by Customer
                ) as C2
          where 35
              > ( select count(*)
                    from ( select Customer
                                , max(PODate) as maxPODate
                             from POHeaderDetails
                           group by Customer
                         ) as C3
                   where C3.maxPODate > C2.maxPODate )
       )
   and H.PODate =
       ( select max(PODate)
           from POHeaderDetails
          where Customer = H.Customer
       )

Caution: untested!