Here's my thoughts on numbers:
Total Population of Eyros: ~8-12 million
Total Population of Eyredyn: ~45-55 thousand
Why? Two reasons. One, these numbers look good to me from a historical standpoint. Two, I think it fits the feel of the setting. Eyros feels like it should be massive. This world feels like multiple armies in the tens of thousands could be martialed by the major powers.
Now, if the condition of things is far more decayed than that, I'd say lets switch it to 3-4 million. That's fairly thinly spread across the landscape, but that's okay. Eyros is not just like the Roman Empire, its also like a very united Confederation. It has been repeatedly emphasized that the provincial management is largely in the hands of individual pillars. So a certain emphasis on local rule is okay. The politics in Eyredyn and foreign relations are often different than the country-man's perspective.
I like giving Eyredyn some weight to its numbers. It can be a terraced city, and it can also be expanding outwards and around the mountain. It feels like it should be a massive city. It is the culture center of the empire, and arguably of the continent.
Just because we want things to be rome-like doesn't mean having slightly off numbers can't be properly explained.
Citizenship in Rome varied across the years. In most of its history, citizens were somewhat rare. That meant they were something of a middle class or better. Non-citizens were not slaves however. Many non-citizens served in the legions, (and upon completion of their hitch became citizens). Many simply served in the auxiliaries for a paycheck. Non-citizens had low-priority but remained an important element in society. There is no reason why a non-citizen might not have access to the Arcaneum. In fact, I suspect serving the Arcaneum for a few years might be a great way for someone of arcane-leanings to gain citizenship and a full wizarding license.
***
Now, there has been some confusion about Mask population numbers. That doesn't seem such a key issue to me, but I'll go ahead and 'fix' it.
Contribution:
It is part of the ancient magical heritage of the Elven people that most, with a spark of potential and the proper training, can be decent at magic. Few choose to walk this path.
Because the Praes Thanatos finds it essential to tie up all Elven spellcasters, they carefully test all Elven children by there 40th year. Out of those children with the potential to work magic particularly well are selected replacement Masks, and additional Masks as ordered by the senate in a vote held at the beginning of every century. This vote usually comes to eight, increasing the number of Masks for every Pillar, the Grand Monarch, and the Guardian Masks by one. The Praes Thanatos push to keep the numbers within reason, concerned that too much magical power should be outside their direct control- even given the amount of control they have over the Masks as a whole.
Those Elven children with some spellcasting potential but not selected to be Masked have their arcane potential sealed within them. Early experiments in simply burning it out resulted in a cancerous sickness that caused unrest amongst the Elven populace and was emotionally traumatic to the Masks.
The Arcaneum, among its other duties, is reponsible for teaching the young Elves selected for Maskhood the disciplines of proper education and log, the beginning theories of magic, the glories of the Empire, and the importance of Oaths. They swear the Oaths at the age of fifty, at which point they are not allowed to see their family again for at least 30 more years, if not forever. They then begin their training, serving as an apprentice under another Mask, an appointed teacher of the Arcaneum, and overseen discretely by a member of the Praes Thanatos.