Hi @Michal257 🌿
You are very much allowed to rent out a room long-term. They write that the lease just has to be for at least 4 month to be seen a LTR.
I think they have made it rather easy to find out what you need to know when renting out here:
https://skat.dk/en-us/individuals/property/renting-out-a-home/renting-out-a-room-or-a-home-you-live-...
The 70 days limit/is when you rent out the whole apartment. When you STR a room (through a platform like Airbnb) and you live there yourself/registered- there is no limit when it comes to how many days you can rent out. You can earn approx. 31.000 DKr/year before you have to pay taxes - but you have to earn less than 50.000 Dkr otherwise you will have to register as a company. This is for STR room rentals via a platform like STR only. LTR got different rules. But I think they have made it easy to figure out on the page I have linked to. If you have any further questions you can easily call them as well.
Best, Sandra