Confidence in conformation search results from molecular mechanics
We have published² a probabilistic treatment of this question, but for the case of MD simulations (and not MC). The principal idea is to use Good-Turing statistics to estimate the probability of unobserved species (ie conformations that have not -as yet- been observed). I have no idea whether this treatment can be transferred to MC, but there is an open source program³ -based on R- that you could try to use.

² https://pubs.acs.org/doi/10.1021/ci4005817

³ https://github.com/pkoukos/GoodTuringMD