How many channels are possible via USB?
Have a look at the current reference manual (page 94 will give you the answer with all the details and limitations).
How many channels are possible via WAVES/DANTE card?
Here, the product pages of the Dante card and Waves card might be helpful to give you needed answers.
using the mixer as a DAW controller -> I know that in that case no audio can pass the mixer.
That’s not true, the mixer can pass audio, only the fader on a layer where midi faders are assigned to are not controlling audio. but the mixer and its mix engine will still work as before.
I don’t know which DAW will work the best in combination with the SQ, since I do not use my SQ as DAW controller at all.
playing music from the DAW and record simultaneously from the SQ to DAW
Yes, the maximum number of signals which can be sent in the one or the other direction. Dante has one feature (for waves I do not know) which might be helpful for you. you can route in the Dante network your signals not only from one source, e.g the SQ, to one target, e.g your computer, but you can do it many to many. So you can use for instance one computer which runs your DAW and another which acts as a Plugin host. This is not possible when using USB.
One last advise: Maybe you should have a real deep look at all the documentation provided by A&H on their web site. That will help you to find the right answers on your own quickly, once you know the content of that documents.
