you can add i think that up to 100 accounts on the same device/phone number

You'll need to log out and then log in with another account. When logged in to one account you can still get Steam Guard codes from the other.

you will do better with desktop auth.

yes

yes it is, i have it, easy and simple switching, no worries, you just add accounts vie the app

Yes, you don't even need 2 sim card. 1 phone number is enough.

I did that, with SDA.

Yes, but note if one account gets vac banned it'll result in vac bans on the other accounts too.

You don't need sim card at all.

Thanks everyboy for help.