Sure, but having only 1 gets you into the Bus factor.

https://en.wikipedia.org/wiki/Bus_factor

But surely there's a balance. 5 or 10 gives you a buffer and saves you money and time hiring the other 90-95.

Until you hit some weird production bug/issue and 2 out of those busy with other issues, one is working on project with tough deadlines, 1 went to a conference and one is on vacation. I don’t count sick days and other stuff.

Recently I had an issue at work which took a day for my colleague to research and I solved it in several minutes because of some low level TCP knowledge. So the broader/higher skills in your tream, the lower the risks. May be those people (who’s looking for a person for 6 months) are trying to lower the risks.

all edits are spelling, I’m terrible at it.

Obviously this is a bit silly to argue about in the abstract, but if there are a number of different domains where 1% problems threaten the business (AWS, security, random other service is down...) and a senior engineer isn't expected to know about all of them, then you'll need some multiple of 1% of the engineers to feel resilient.

Rough amount: number_of_domains * max(ceil(1% * workforce), bus_factor_min))

At small companies, that may be most engineers. At larger companies, it may (or may not?) level out to a more reasonable ratio.