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.

Why might one need the second bus - a capacity calculation

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.

Other Options

There are other options to get to the Warner Bros Studio from Watford Junction station, in case of unforeseen situations:

• Buses 8 and 10, operated by Arriva bus, departs from Watford Junction and passes near the Warner Bros Studio.
• Taxis between Watford Junction and the Warner Bros Studio cost around £12-13 one-way based on personal experience travelling to nearby places, and minicab hires cost less (which is a viable option if you have a e.g. group of four).