The press material issued around the time of the order stated clearly that eight Talgo sets were for Hamburg-Copenhagen. But if my maths are correct, the pre-COVID diagram (three roundtrips a day, plus one overnight roundtrip during the summer) could by covered with 4 or 5 sets. Therefore the eight sets ordered should be enough to cover both Hamburg-Copenhagen and Hamburg-Aarhus.
EDIT/automerge:
As my maths teacher used to say, always show your working. So, reading off table 50 in the summer 2020 European Railway Timetable, here's the peak summer season schedule, which can be covered by four trains.
IC399: Københaven H 0005 / Hamburg Hbf 0624 >>> IC396 Hamburg Hbf 0853 / Københaven H 1333
IC393: Københaven H 0726 / Hamburg Hbf 1202 >>> IC394 Hamburg Hbf 1253 / Københaven H 1733
IC395: Københaven H 1126 / Hamburg Hbf 1602 >>> IC392 Hamburg Hbf 1653 / Københaven H 2133
IC397: Københaven H 1526 / Hamburg Hbf 2002 >>> IC398 Hamburg Hbf 2356 / Københaven H 0655
Outside the summertime peak, IC397 overnights in Hamburg and returns as IC396.
Second EDIT...
Aarhus is a bit harder to read from the tables. IC382 is summertime only, but isn't matched by an equivalent summertime only service in the opposite direction, so I presume that maybe IC385 is doubled up in the summer and divides in Hamburg to serve both IC382 and IC386. The rest of the year I presume it stays overnight in Hamburg to return as IC386.
IC383 Aarhus 0937 / Hamburg Hbf 1402 >>> IC384 Hamburg Hbf 1450 / Aarhus 1918
IC385 Aarhus 1337 / Hamburg Hbf 1802 >>> IC382 Hamburg Hbf 1901 / Fredericia 2212 and/or IC386 Hamburg Hbf 1053 / Aarhus 1518?
So it seems obvious that eight trains will give DSB enough capacity to use the Talgos between Aarhus and Hamburg as well. That's not to say that with the new trains (and in a decade or so, the new tunnel) DSB aren't planning to increase frequency...