Buying Xenforo licence


Hi all,
I am interested to buy licence and start to build my forum,
but I didn't understand exactly the pricing .
So if I will have for example :
General Topic

Another Topic

Does that means I am paying 2 licences ? Or 6 ? Or just one , because it's a 1 site , one forum instalation ?
Can one forum have for example 30 main topics and in each topic few subtopics ? Is that one licence ?

Thank you .
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