I second what Mike and Jaya have said.
And so do I.
> if JY say that ‘the max is 20 for the main tour and 15 for the add-on’ then most people would interpret that as there being 20 places in total - 15 of which can take the add-on option
This seems to me to be the only reasonable way of interpreting the figures: an "add-on" is precisely that: a subset, not a separate party!
> if people are in any doubt about the maximum group numbers for a particular tour, they consult the brochure which does show the correct figures
And why should one have to consult the brochure to interpret the website??! How would one know which of the two was correct? Surely the two should always be consistent, whatever the wording used?
> group sizes should be a maximum of 25, and judging from replies JY have received on this subject, I believe that view is shared by the majority of their customers.
For me, even 25 is on the large side: most of my recent trips have been to "minority interest" destinations not offered by JY, so there have always been less than ten participants: this works fine (one can really get to know one's fellow-travellers) - and seems to be commercially viable (ie profitable;-): presumably the reason why JY tries to maximise group size?
Jim