I directed a community theatre production a few years back. Because this was small town community theatre, I added an ensemble of three. They, along with the actor who voiced the puppet from a microphone in the booth, were in "Skid Row." They each played one of the three visitors in "The Meek Shall Inherit." One played Skip Snip at the end, and they other two served as puppeteers.
There are usually four puppets: the first one just wilts, if I remember correctly. The second was controlled by Seymour, and I used two different performers for #3 and #4, just because it was SO hot inside the puppets, I didn't want one person inside them for 1.5 hours. The puppeteer who was not controlling the puppet helped set the puppet during the shift, get the other puppeteer safely inside, and help pull actors though the puppet while they were being eaten.
We rented our puppets, and they arrived a few days before tech. We dedicated an evening to puppet rehearsal-- getting the actors used to manipulating it, then adding the voice, then adding the actors who get eaten. It went pretty smoothly. The puppet that required the most rehearsal was actually pupptet #2, because it was controlled by Seymour's arm while he is singing.