this is easy, really but you never stated what PCs
makes and models.
nor what UPS you have, I've worked on UPS in 911 centers, yes, monsters.
rooms full of huge batteries, yes.
but in your case it's called a power budget.
there are 2 things
1: TOTAL WATTS. (demand)
2: AND RUN TIME NEEDS, 1 MIN , 1 HOUR OR WHAT?
What I do is buy a watt meter, (or ac amps and do the simple math)
then measure all PCs power needs.
add them up and buy a UPS with 2 times that.
when power fails turn off the PCs, in the safe way.
the manual for each UPS made (RTM read it?)
shows watts demanded and hours run time.
that be all you need right there.
also what country are you in.
some country's have brown out conditions, not just failure.
and very tricky that is for any UPS to work right, (some can)
amps demand is here (x120v = watts (VA)
to use such a meter,
we break out the AC black wire.
and clamp on only on this black hot wire, it the meter will not work.
I have this device, or make one by hand. (1 plug,1jack 3 wires)