Scaling how often I have failures on my 80 servers, of which 50 are similar in spec to Ahrefs (16 HDDs), it should still be less than one full-time position to manage 850 servers. Probably much less, as I'm a bit inefficient when it's <5% of my job.

I have a fixed day or two each year working out what to buy and getting quotes (which would be the same time whether it's 20 or 200 servers), half a day per 20 servers for installation in racks, and call it half a day per 20 for initial configuration (most of the time to script the process, so it would also be similar for 200). After that, it's at absolute most an hour every couple of months to deal with a failed disc or similar.