PubMed grew beyond 14,000,000 abstracts in 2006 (by the end of 2007 the 17,000,000 mark was passed). In 2006, UMLS contained well over 1,300,000 concepts. Only 185,262 concepts from UMLS were actually mentioned in PubMed (2006 version) and, therefore, the concept space of the entire PubMed corpus could be captured in just over 185,000 Knowlets.
Mons et al. Genome Biology 2008 9:R89 doi:10.1186/gb-2008-9-5-r89