If they're low-mid end PCs, you can plug atleast 5 (this is assuming a max draw of about 500W per machine, which, unless your running multiple gfx cards, is an over-estimate)
As long as the total current draw through the adapter doesn't go about about 13A you should be fine (assuming your strip adapter has a 13A fuse in, as nearly all do) if you exceed this, all that will happen, is the fuse in the plug will be blown, if this happens, unplug at least one machine, replace the fuse, and try again. The fuse is there for safety reasons, so you won't be able to connect more items than is safe without blowing the fuse first.