When there's a group of musicians playing, it's often not practical for several of them to stop what they're doing so they can clap instead, so is there an acoustic percussion instrument that can be played by one person, that sounds like ensemble handclaps, i.e a group of people clapping together?
I don't think I've encountered an acoustic instrument as a percussionist that perfectly emulates a hand clap sound. Typically you could use something like a small whip (two boards hinged together), or large castanets to provide a similar effect.
SONOR apparently makes a 'hand clap box' that is supposed to sound like hand clapping and would be playable by a single musician (Although personally I think it sounds too close to a woodblock):
Not sure what you mean by "ensemble" hand claps, though there are a few ways drummers have tried to emulate electronic clap sounds. Basically, they boil down to various rim-flam techniques. You can experiment and adapt whatever sorts of drums you have available.
I wanted such an instrument to cover some pop songs tomorrow, e.g. Macklemore's "Thrift Shop" at 00:30:
I also couldn't find anything I could easily buy at a music store, so I experimented with household items. I imagined maybe a folding paper or plastic fan might work, but I didn't have one.
But I found you can get pretty close with half a deck of cards. Hold the half-deck in one hand and use the other hand's thumb to do a controlled snap on the short side, like a flip book but really fast.
It's easy to improve technique by playing around with it. I've found that if you hold the deck in a shape like, um... remember those old-school rubber erasers?
If you make your deck look like that, you add on more choral "voices" (separation between claps), and sounds even more like an ensemble clap.
Of course, not loud enough by itself but I plan to hold it near a microphone.
Though you do specifically ask for acoustic percussion instruments, I'll answer off topic that, in terms of faithfully simulating a sound, an electronic sample would be very effective and there are hardware form factors that would work nicely in a live, acoustic group setting: pad or button to trigger, built-in speaker, etc.