It seems, that „allow instrument changes” option occurs only when I have one instrument change on a single system. When I have more than two different instruments, Dorico is showing it on a multiple staffs.
How can I get single staff on a one system, when I have more than one instrument change?
Provided none of the music belonging to the various instruments held by the players actually overlaps, then Dorico will happily show two or more instrument changes on the same system. There must be some music that is overlapping. Feel free to attach the project so we can take a look.
Thak you, Daniel, it works indeed. After your answer I realized, that I asked the wrong question, sorry for that, I was a little bit tired.
What I really want to say is, if I have more than one instrument change on a single system and the music overlaps in only some instruments, Dorico don’t allow „instrument changes” to rest of the instruments.
See the example:
– „rins 2” could be on a same staff together with „tpbl 2”.
– „tom-toms” could be together with „gran cassa”.
– „rins3”, „tam-tam” and „bell tree” could be together.
Instead of 8 staffs, I could have 4.
Is it possible?
Judging from only this small example, I’d say this is complex enough to warrant the attention of turning into a grid, which would aid the percussionists’ reading. Of course, it might make things more complicated for you, or unnecessarily complicated in the rest of the piece, I don’t know, but it’s something you should consider.
Get back to work, Luís 
Thank you, LSalgueiro, for the advise. This is only one example. I would like to know, if the „instrument changes”, I described above, are technically possible, without ceating percussion kit.
No, instrument changes end up as a binary choice: either they can be applied for all instruments, or no instruments. You might find that assigning these percussion instruments to two or three players and then assigning all of those players to the same part layout gives you greater flexibility, though of course instruments cannot move between players over time.