within in the project or within all of iNatuarlist?
in the Explore Page, you should be able to see the top 1000 most observed leaf taxa. so if the project has fewer than 1000 taxa, then you should be able to scroll to the bottom of the list to see taxa observed only once within the project.
if the project has more than 1000 taxa, you’d either have to partition your set into smaller subsets, or use code or some sort of tool that can retrieve records from the API beyond that 1000 limit and/or sort ascending instead of descending. the 3rd tool described here (https://forum.inaturalist.org/t/several-external-tools-for-inat-data-by-kildor/19906) will allow you to do do the former, but it only answers your question in the context of within the project.
if you need to answer the question in the context of within all of iNaturalist, then you have to use your own code or another tool. here’s an example that might help: https://jumear.github.io/stirfry/iNatAPIv1_observations_species_counts?project_id=123&order=asc. it includes both count of observations within the project and within all of iNaturalist.