Hi stempest,
there are no general rules on "how many grogs there should be, or setting out any kind of normal(ish) distribution of roles". You can configure your covenant as you like, using Covenants or not using it.
There are just too many possibilities, and too many different styles of play and of distribution of roles: just magi, magi plus companions, magi plus companions plus shield grogs, and so on.
Best decide first with your troupe, where your covenant shall be, what kind it is, and how it relates to the country around. Covenants p.6ff Boons & Hooks helps here. In due time you find how many grog level people you need, and what they will be doing. And then Covenants p.42ff Covenfolk comes in handy.
Here's a fan-made covenant to start out from, with the explicit purpose of being general and vanilla: https://forum.atlas-games.com/t/vanilla-covenants-project-returns/10436/1 .
Here's an older one published by Atlas: atlas-games.com/pdf_storage/Semita.pdf .
An older sourcebook with a splendidly detailed setting and covenant is ISBN 1-887801-85-5 Triamore: The Covenant at Lucien's Folly.
And a new sourcebook with five detailed covenants is Through the Aegis: atlas-games.com/product_tables/AG0311.php .
Cheers