Well- it depends on a few things.... how big the structure your building is, how many side projects you want to do, are you having a kids team or food team to help with kid projects or manage the food/drinks for the day. We built two structures (one was large that was for 75 kids and the small one was for about 50 kids) we had 200 people. We had a kid team with 15 people, safety team with10 people, logistics with 8 people, food with 15 people, 4 runners, a cement team of 6 people, side project of a landscaping team 15 people, another side project team of 20 people to do map of the USA and other painting projects, the rest broke out into teams between our 2 structures. The kaboom tool kit has a formular to figure it out and your playground rep maybe able to suggest a number that is right for you. Also fiigure about 10% may not show up. Many just want to be there to experience the day and help out for a short time and some stay all day. Our rep would have been happy with 125-150 but this was good so all the side projects we wanted got done. Good luck.
As a project manager with KaBOOM!, I'll say that our builds typically involve 150-250 people. But this number includes, in addition to just putting together the playground pieces, many enhancement projects such as picnic tables, benches, moving wood fiber for the surfacing, mixing concrete by hand, etc. So, as someone previously responded, it depends on a lot of things. How many playground components/pieces are there? Do you want the build to be over just one day or a week? How skilled are your volunteers? What type of surfacing are you using and are volunteers installing (ie. rubber) or moving (ie wood fiber) this surfacing or will it be done by a contractor? Are you doing any enhancement projects as well?
Talking just putting together playground pieces, I would say 50 is a good number assumming your playground is a fair size - (perhaps 15-20 active play components). For KaBOOM! led builds, we'll make 4-5 groups/teams. For example, take a look at the instructions for each piece and separate them into 5 relatively even groups (A,B,C,D, and E). In other words, you may take things like a slide and swings that will each take a long time and put them in DIFFERENT groups. Keep doing this until you have 5 even groups with some more elaborate components and some simpler ones in each group - perhaps about 4 pieces in each group. Then, divide up your 50 people into 5 teams of 10 (A,B,C,D, and E). Team A would of course put together the all the Group A components and so on. You could always take a look at your components, separate them into groups and then use that to help you figure out how many people would be a good number. As a general rule, about 10 people would be a good number for each group of components (again, roughly 4 pieces).
Again, of course there are many factors, but hopefully this provides you with some guidance!!!