Well to try and get it back on topic, it is important to note the map Alxcj linked. The ancient Nordic Empire spanned a far larger distance then Skyrim. That being noted there is definitely a correlation between the nine masks of the dragon priests and the 9 divines of the nords.
Only after installing the Dragonborn DLC,,, the eight in Skyrim proper unlock the ninth of Skyrim proper, The four additional mask you speak of are not even located in Skyrim they're in Solstheim which of course is NOT Skyrim although once held by Nords ot was not part of a hold and ruled in itself - who knows maybe it was four separate states in it's time?
ASIDE from that nine priests and masks does not mean nine holds ... the dragon priests have nothing to do with the holds.
"Of course! Nine is the most basic of magical principles. Oh, how stupid of me." - to quote Wylandriah,
"By the NINE", "Nine divines protect me!" ,,, may I go on?
It's quite simple... and is natural in scheme, NINE is an arbitrary number seemingly picked by the developers as a representitive theological point, kind of like the bible uses 40 and/or 6, stems from the human rational to place some context or meaning in mathematical terms. We are counters.;)
In light of this nine is the number of dieties once recognized by the empire (shall you include Talos), They could have included Shor and others but limited to nine as comprimised by way of assimulating mer and man.. of course now turned to be eight... given Thalmor plan of insurgance.
Nine holds play into the scheme of the civil war... could have been any odd number really but four per side and a neutral in the middle of it all played a decent number given the map space and ideas behind such a war.
NINE masks in Skyrim... again is just a number is obvious were nore dragon priest than that but nine dragon named were well known ruthlessly,,, The priest of Solstheim of course as I mentioned earlier are not in Skyrim. Interesting point though... you see there are 13 named dragons Alduin Durnehviir Krosulhah Kruziikrel Paarthurnax Odahviing Mirmulnir Relonikiv Sahloknir Sahrotaar Nahagliiv Vuljotnaak Vulthuryol if you have all the DLC, but when you pull out those in the DLC's ... you get nine.
SO... the masked priest are nothing more than the high-priest of each dragon ... as you may see.