I must admit I'm not really enthused by this question, as it stands.
The knowledge of the molecule with the most number of atoms is perhaps of passing interest ("oh, cool") but is not particularly useful to anybody.
Perhaps more importantly, I assume we are only counting species that have been documented in literature — otherwise it would get out of hand.
But, bearing in mind that almost any designer molecule can be made nowadays (including Very Large Things such as modified biological macromolecules or molecular machines), such a distinction is kind of arbitrary: it doesn't illustrate a fundamental scientific limit or a concept, but rather it illustrates what people have bothered doing so far.
I would suspect that whatever record we find could — at least in principle — be modified to include a new atom simply by swapping a hydrogen out for something else, for example. Thus the "record" that we find might not exactly be a triumph of scientific progress but rather a mark of where us humans got lazy (or started thinking about research that was actually useful).
But: I think we could make something interesting out of the idea of having lots of atoms in one molecule. I am very biased by virtue of my NMR background, but having many different nuclei in one molecule is (or was) of interest back in the days when NMR quantum computing was still a thing. (This, for example, led to the experimental realisation of Shor's prime factorisation algorithm.)
Instead of asking "what's the most nuclei we have ever crammed into a molecule", how about changing it to something like: "Have people specifically sought to make molecules with many different nuclei, and why would they do so / do they find use in chemistry research?"
The fundamental idea is the same, so it would satisfy the less technically inclined crowd who are happy to just go "oh wow, 10 different elements, cool!". But at the same time, it'd also be a much more useful and interesting question, in my opinion.
