I remember when I was a teenager a friend asked me why chefs start work at 8:30am if lunch is at 12:30? At the time blogging wasn't a think, so 30 years later here you have it. This is a typical Saturday.

6am: Walk the dogs...I take it some van operators won't have dogs, but hey... it's a 5000 steps walk!  And this is after a busy Friday night.
7:15am: after a quick breakfast I head to the restaurant to make dough for the next day, about 50kg of it, which will yield around 180 dough balls/pizzas. This takes about 2 hours
12 noon: Start loading the van with logs for the fire, fill the water tanks and give it a clean if it hasn't been used for a few days. This from time to time includes washing the outside as well.
12:30: Start the fire. Because it's winter and the van is not in use every single day,  and we have a biiiigggg oven (1m internal diameter and around 400kg in weight) it takes time to bring it to the ideal temperature, which is about 400 degrees. This means that once we arrive at the venue, we just need another 30 to 45 minutes to be ready to cook
1:30pm: After a quick lunch we head to the restaurant to pick up dough, toppings, sauce etc. Because once we are out, WE ARE OUT! we need to triple check we have got everything we need. The team at the restaurant will have done most of the prep, but everything needs to go into coolers and then be loaded in the van. Everything needs to be packed tightly, and all boxes need to be places on the floor, so when we arrive at the venue and open the van's doors it doesn't look like a bomb has exploded! My worst nightmare is finding the tomato sauce spilled all over the place.
3pm: If service starts at 4pm, we want to be there at least an hour before. Earlier if this is the first time at this venue, as in this case we need to ask where to park, where we can connect for electricity etc. First thing is always the oven. Take advantage of whatever residual heat we have left and start a big fire. Followed by preparing our station, folding boxes, organising pre orders and generally getting ready.
3:45: TA DA! At least in the winter, the service hatch only goes up 15 minutes before opening time. Why? because once we open it it's like being outside and it's cold... very cold :(
4pm to 8pm: We serve some cracking pizzas, chat to people and generally try to have a bit of fun.
8:15pm to 8:30pm: This is the disappointment and "oh no" area. We have started cleaning down and now the oven has got it's door on, which means the fire is dying a we can no longer cook pizza, even if we wanted to. This is when customers seem to arrive in numbers and ask the rhetorical questions "are you still serving" or "are you still open". 
8:30pm: After disappointing more people in 10 minutes, than we have served in 4 hours, we start cleaning down in earnest. Put everything away, close the hatch, unplug all cables, make sure all sort of bolts and locks are closed tightly and then wish our goodbyes and drive back.
9:15: Get back to the restaurant, unload any left overs, store anything that can be reused in fridges, throw the rubbish away, reverse the van without hitting anything and finally drive home
10pm: Home time!

This is a question we get asked a lot. Of course this is not a precise science and Google hasn't created an algorithm for this yet! But here are a few things to consider that may help.

1. Is pizza your main meal? If you are having just pizza and nothing else you may want to got for the safe option and order 8 or 9 full pizza for every 10 adults and half a pizza for every kid. More on this in the next question. On the other hand if pizza is your "evening food" than it's a safe to assume that a smaller percentage of the guests will want some and they may just eat a few slices, say half a pizza.
2. What's your guests mix? Do you play rugby and most of your friends need 5000 calories a day? Or are your friends more of a standard mix? How many kids are in attendance? If none of your guests skews the average, then refer to question number 1. Age may play a part as well, as naturally older people tend to eat less, especially late in the evening.
3. What time is pizza being served? Is pizza is being served close to lunchtime or early afternoon, people may be very hungry. On the flip side of the coin, if you are hiring a pizza van for the evening, after the weeding breakfast has been served, you may find that your attendees are not that hungry and may just "nibble" on it.

Let us try an put some numbers down. Assuming a typical crowd of 100 people, comprising of
35 ladies - 3/4 of a pizza each
35 man - 1 pizza each
10 kids between 5 and 12 - 1/2 pizza each
20 between gran parents, aunts and uncles - 3/4 pizza each

Total: 81 pizzas