In what percentage of tooth #14 (maxillary left first molar) cases will you find 4 canals?

Maxillary first molars (#3, #14) have 3 roots: mesiobuccal (MB), distobuccal (DB), palatal. The MB root commonly has 2 canals (MB1 and MB2) — seen in approximately 90-95% of cases. So 95% of max first molars effectively have 4 canals (MB1, MB2, DB, P). Always look for the MB2 canal during RCT of upper first molars!