Security aren't judges.

Events where the players are likely to be playing a format they don't play
often will need more judges. That could be standard, draft or vintage.
Inexperience in the format causes work for judges

I'll just add, if you're running multiple events at a venue, factor for additional L2s to cover those. I.e. at our PTQs we'll sometimes run a GPT that starts after round 3-4 so people who drop from the main event have a “second chance” at a big event. Throw on another L2 to head judge draft pods, mini-masters, etc…

I like to see a 2:1 ratio of L1s to L2s. These events are great training grounds for L1s aiming for L2, so I try to maximize the number of L1s. Just make sure each one is assigned directly to an L2, so if they need to consult they have someone right on hand. It's also good training for L2's practicing as team leads for L3! :)

I'd agree that there tend to be fewer calls when the players have more experience with the format and common card interactions. This is why Limited events usually need more judges, and why Legacy format events vary in terms of staffing needed.

Emphasize to your TOs the idea of investing in judges, because the more opportunity there is for mentoring the better we get, and the better the event is perceived from a player standpoint. This encourages players to play in more events, buy more product, and bring more friends.