First time when apply you get 90 days single entry. After that you can apply for 1 year multi. If you want an extension instead of leaving the country you need thaistaff and some other requirements.
So theoretically if you only want to setup a company you need a visa for conduct business BUT you need a workpermit to sign documents (this is consider work) so the best option if you only want to open company or work is the first option.