Content agencies quote in posts per month because it is easy to price and easy to deliver. It is also close to meaningless: eight posts targeting nothing in particular will lose to two that own their search terms, and cost more.
The real constraint is your keyword map
Volume should come out of strategy, not the other way round. If your keyword research identifies twenty commercially valuable terms you do not yet cover, that is twenty pages of work. Spread over a year, that is fewer than two a month. The number is an output, not an input.
Updating usually beats publishing
An existing page with history, links and some ranking momentum will often move faster from an update than a new page will from scratch. Most sites with a few years of content have more value locked in their archive than in anything they could publish next month. Audit before commissioning.
Quality has a threshold
There is a level below which content does not compete at all, and it is higher than it was, because search engines have got better at recognising volume production. A piece that is thinner than what already ranks does not rank a bit lower; it usually does not rank. Publishing more of it does not aggregate into authority, it accumulates as dead weight.
A sensible default
For most small and mid-sized businesses: two well-researched, genuinely useful pieces a month, plus quarterly updates to existing pages. That is enough to build a cluster within a year, and slow enough that each piece can be good. If you have in-house expertise and capacity, more is fine, provided the standard holds.
When higher volume is justified
Large sites in broad categories, publishers whose business model is traffic, and businesses attacking a well-mapped cluster on a deadline. Even then, volume works because each piece has a job, not because there are lots of them. Deciding which pieces have a job is what a content strategy is for.