You can put multiple pork butts on the smoker as long as you leave a couple of inches of space between them. If they are too close to one another, they will take longer to cook.
The biggest factor on cook time is whether you decide to cook the butts with or without wrapping in butcher paper or foil. If you decide not to wrap them, they might take several hours longer, but you will get a better bark if you do not wrap.
The lower the cook temperature, the longer the cook time and the more smoke can be absorbed, but there is a point beyond which no more smoke will be added. If you need to speed up the cook time, you can always wrap the butts in foil and turn the cook temperature up to finish off the butts. The main thing is that they need to reach an internal temperature of approximately 203F and then have an hour or more to rest before you attempt to pull, chop or slice the meat. That rest time allows the juices to redistribute throughout the meat fiber.
If ayou are cooking pork butts to feed a large crowd, I suggest you consider cooking them ahead of time rather than waiting until the day of the event. Pork can be kept in the refrigerator for a few days and then be reheated. If you need to keep it longer, it freezes well. Having a crowd of hungry people waiting to be served can be quite stressful. Do as much as you can ahead of time so you can enjoy being with your guests rather than wondering when the pork will be ready.