When elections come around each year, I'm just wondering whether this site has a ratio of moderators to users (i.e. 1 moderator for every X number of users) when determining the number of moderation positions required. If so, what is the ratio? If not, how on this site specifically do we determine the number of moderators there should be?
I don’t think so.
In a Meta.se question about moderator elections, it’s explained how a site comes to have more moderators:
The community team periodically looks at the work load on graduated sites and will reach out to the moderation team if it looks like more help might be needed. Broadly, it's up to the moderation teams to indicate whether they need additional hands, and how many sets of hands would be ideal.
The “community team”, also known as the Community Managers or CMs, are Stack Exchange employees who are responsible for making sure the sites run smoothly, which includes running the moderator elections.
That answer goes on to highlight that elections are not held annually; only when an existing moderator steps down, or when the moderators want more hands (in the recent SFF election, it was a combination of both):
Although we'll reach out to moderators on the anniversary of their last election, just to see how they're doing, elections do not always take place at the same time every year. Sometimes a year can even go by without any election at all. Most elections are scheduled as a result of moderators reaching out to Stack Exchange to ask for additional help.
Moderators-to-users would be a poor metric for determining the number of moderators. We have a lot of drive-by users – particularly for story ID questions – who add minimal new work for the moderators. Most of the work (editing their question, etc.) can be done by regular 10K users.
More users != need for more moderators.