I think it depends on the shop. I have managed a 7x24 operation
(computerized weather data feeds to media clients), and when we did the
calculations it turns out that you really need 5 FTE (full-time employees)
to cover staggered shifts, weekends, vacations, sick days, etc. OTOH, I
have heard of shops that have only one sysadmin with a pager "surgically
implanted" (just a joke... I hope?). If there are ANY (persistent)
problems, that poor fellow will soon become suicidal. It probably depends
on how active/chaotic the operation happens to be. Are they doing heavy
development and/or modifications? Growing and/or expanding?
For a small mom/pop business that runs a few machines (not 7x24 with
99.999% availability, or anything "silly") then you could probably get by
with one solid sysadmin, with a "deputy". Plan to spend some extra $ on HA
(cluster, RAID) and automation (tape libraries), and bonus(es?)! YMMV!
BTW, if you're in a large shop, and the lowest on the totem pole, you
might end up getting lots of night shifts or weekends to work. That can
isolate you from (some) humanity. You might have to hang out with shift
workers, or get used to solitary "quiet, personal time" (i.e. read, learn).
FWIW, if you're going to do the "isolation" thing, you might as well make
it worthwhile: find a job somewhere remote, so you get high (isolation)
pay. Sometimes there is danger associated (danger pay?). Choose priorities.