When people talk about ontological parsimony, it is generally used to add or subtract credibility from a theory about what exists 'within' the universe or multiverse. For example, it's used to discuss the likelihood of different scientific theories being true or the existence of certain particles or the existence of supernatural beings etc.
It makes sense to me in this context that the parsimonious solution is the most likely because the more parts there are to a theory, the more failure points, and the more points that may need to explained by chance events and coincidences. To use a silly example, if you claim that atoms are held together by elves rather than just fundamental forces you have to explain how the elves function, how they came to get into the atoms, why they hold them together, etc.
However, is a parismonious theory more likely to be true when we talk about pre-existing objects, things that exist without cause?
Leaving aside the question of whether anything can exist without cause, I'm not sure why the parsimonious explanation would be the most probably here. It seems intuitively true, but I'm not sure that doesn't just come from a perception of the world we live in, where parsimony is a useful guide.
People have posited various objects existing without cause before the universe existed - for example - a singularity, a single particle, a God, pure consciousness, a multiverse, all of mathematics in physical form, etc. I'm not wanting debate how plausible each of these are but if it is really the case that the simplest of these is the most probable. It seems to me that by definition, probability weights up the likelihood of different events, or different cases caused by different evets, and events happen over time and have causes. If something has always existed without cause, I'm not sure simplicity would really make it more probable.
Any answer very much appreciated! I'm genuinely not trying to push a particular theory, I'm just confused about parismony itself.