If you have three good knowledgeable people, you can do a small dressage show. (Scribe, back gate person, secretary). Signs can do your parking and direction. Entries online as much as possible.
If you have five or six (or more) good people, you can do a jumper ring. Scribe, in gate, warmup, and 2 for jump crew or so. The more you have for jump crew the better it is for all the crew, as they can spread out, and cover areas of the ring without traversing the entire span – keeps them going a bit longer during the day. Having a couple of back gate/ in gate folks also helps, run them in 3 to 4 hour shifts if you can or pair them to learn to help each other as a team to keep rides moving.
An event takes the most people. In addition to the above, you need a starter for XC and finish timer. And jump judges….at least 20 for most recognized events. If you are running a starter event you really only need judges that can be stationed to see multiple jumps in their area. Olde Hope Farm used to have a tower that allowed the jump judges to see the entire jumping field. At Plantation, at the top of the hill that traverses the field, you can see most every jump. And you can always go on the honesty system, too, where the rider reports to the finish person if they had a stop or not. Yes, this works!
The point is, events take quadruple the amount of volunteers that other equestrian sports need to function. Because of this need, the sport has to continually utilize outreach to find volunteers to fill necessary jobs. A factor is the jump crew syndrome. That is, a bare minimum of help makes it harder for all who do show up. Better to have more than less, in order to give everyone collectively a kinder experience. So you can’t really get by with a bare minimum. It will erode your diehards, and then you will have nothing.
To jump crew for eight hours, to run warmup or bit check for eight hours, to scribe for eight hours is BRUTAL. Every single competitor at horse trials and events needs to know in their bones what it takes to survive an eight hour day volunteering. JMO.