Summary: One should be prepared to wait for the second bus if they are visiting during school holidays, Easter being one of them. There are other options to get to the Warner Bros Studio from Watford Junction though.
According to the official booking website the allocated time slots come in half-hour intervals, and according to this tour review, each slot takes in around 150 people:
The tour works like this – once your allocated time-slot arrives, you join the queue to go into a presentation room, about 150 people at a time.
… which matches my personal experience on how many people that room can fit.
The shuttle bus operates every 20 mins from Watford Junction:
Buses run every 20 minutes from 9.20am (with additional services available from 8.15am when the first tour is at 9am).
The coach operator uses the two-axle variant of Enviro400s, which has a capacity of 70-90 people judging from the neighbouring London buses of the same model. While such arrangement means the capacity of the shuttle bus would roughly match the demand per hour, the unaligned timetable means there will be more people queueing at certain minutes past the hour.
The situation is more pronounced during school holidays (when the OP planned to visit) and one should be prepared that they might need to take the second shuttle bus.
There are other options to get to the Warner Bros Studio from Watford Junction station, in case of unforeseen situations:
This depends on the time of year, day of the week, holidays etc.
But, generally, yes there will probably be a small queue. When I have been before it was during a week day, we arrived about 10 minutes before the bus left and there was maybe 5-10 people ahead of us, when the bus left on time it was mostly full.
To be on the safe side I would include some time to allow you to get from the platform to the right bus stop (a few minutes at most), and then some time incase the first bus to arrive does fill up, but for the time you have chosen as long as it is not a school holiday, you should be fine.
Edit: due to it being the day before easter, and easter holidays for many schools, I would arrive early and be prepared to take the second shuttle as there could be queues.
Credit:stackoverflow.com‘