We use Elasticsearch and keep their recommended limit of 10k records, which is why you’re hitting this limit. You can see the GitHub issue here: https://github.com/inaturalist/iNaturalistAPI/issues/134 We should, however, have an error message when this happens, I’ll make an issue for that.
The way to get around this would be to use more filters, such as filtering by taxon, dates, location, etc. For example, searching for all reptiles in Europe that Need ID returns just over 1k records: https://www.inaturalist.org/observations/identify?iconic_taxa=Reptilia&place_id=97391