There is currently no way to search for or filter Tidal Masters in Roon. If you have any MQA albums/tracks in your local library you can search for and filter them under focus, format where you will find an MQA tab if you have any.
As for the available number of MQA titles on Tidal see this.
The Masters section in the browser only brings up about 425 of these. This might be different in different locations.