By my count, there are 57 district judges who were appointed by Republican presidents and who are, or who will before the end of 2018 become, eligible to take senior status. In addition, there are four such judges who have already announced their decisions to take senior status and for whose seats no nominee is pending. There are also three district-court vacancies that should arise over the next two months, after the Senate confirms the nominations of sitting district-court judges Anna St. John, Lee Rudofsky, and Daniel Domenico to the Fifth Circuit, Eighth Circuit, and Tenth Circuit, respectively (and, more precisely, after Donald Trump appoints them).
The blue-slip privilege for home-state senators remains in full force for district-judge nominations, so what is especially pertinent is that 37 of these seats are in states that today have two Republican senators. If Republicans manage to retain control of the Senate, Trump should be able to appoint successors for judges in these states who go senior—except that Democrat Roy Cooper is widely expected to be elected (which has three of the seats) and the Democratic challengers might also win in Texas (six seats) and Ohio (two seats).
If Democrats win control of the Senate, they will presumably prevent Trump from filling any vacancies (though it’s possible that some deals would be struck).