The first had a small team (< 5) of also highly paid senior developers like him to build and maintain a set of systems. I wouldn't call it "manage" so much as "coordinate colleagues". The second was working alone.

In both cases the work was difficult to do, difficult to share across a team (solve a few hard problems well rather than cycle through hundreds of small JIRA tickets) and highly value adding for the company which is why they were willing to pay.

Ideally zero as they're on an "individual contributor" track where they're focused on engineering. They may have other people who they mentor but no direct reports.

None? Some companies have individual contributor tracks that end up as positions like "senior architect".