Provencal also has Guillermo of Mercere, a rather brash member of the Cult of Heroes and certamen champion (F&F pg 112). Again, no stats - I think the only statted Mercere magus is the template in the corebook.
Are they really that popular?
In all my years I've never had anyone want to play one. One saga we decided to make a covenant with only representatives from 2 Houses, in an attempt to make House plots more useful by activating several magi. we let the saga's novice choose the two houses, with only a cursory knowledge of them. He chose Mercere and Merinita, and two of us designed a pair of Mercere twins sent to Loch Leglean tribunal to take control of the Redcap network and "civilize" the area.
And in all my years of sagas before/without Trinunal books we had but a single Gifted magus Mercere defined as ncp.