Tills often are low demand, they are typically touch LCDs, low spec processing, low tech in general, with a few ac/dc adapters, KVMs, etc, prob 40a for all sockets all at one time. To be safe, two rings, high integrity, absolute max 32a per ring, 20 may be OK.
If they are actual PCs, or Raided servers, then 2 x 32a or 3 x 20a, but I would be surprised if they used that. RCBOs are probably a good idea, they WILL trip a single RCD at some point
Fit a current monitor, it will encourage them to power off at night.
Anyhow, get a look at a typical station, ask what a typical sale is, add it up, double it,compare with the number you first thought of, bingo.
A typical low to mid range PC has a 145 to 240w SPSU running at under 50% when up, an LCD screen, add an AC/DCs adp for addons, a double socket is looking at 2 to 3 max. I have seen 300PCs with old CRTs being commissioned in a 6m x 6m classroom by 5 guys, over a few hours, on a single ring shared with another room. The breaker went once, and that was a duff SPSU.
Get the kit out, stick your clamp around a modded lead.