Seeking advice for bus transportation

Hi all,
we'll be visiting your lovely city in about two weeks and we need to go from Central Station to Attractiepark de Waarbeek.

I saw that there's a bus going 2-3 times per hour and I'm trying to figure out the cheapest way to use it for around 15 people.

I noticed the 9292 app, but it let's me only buy 9 tickets at a time. Is this the usual way to go or are there better alternatives?

Thanks in advance!

(I know it's not that far, but this is a very lazy group)