The most blades I've seen are 8 on the Mi-26.
But the highest theoretical lifting efficiency is achieved with the fewest blades and experiments have been done with single blade rotors (with a counterweight - there were vibration problems that couldn't be resolved).
So in practical terms, the most lift for the least power is achieved with a 2 blade rotor, but the need to absorb more power with a limited desirable disc diameter forces you to add more blades (or you may want a smaller rotor with lots of blades for maneuverability or other reasons, but it's sub-optimal from a horsepower efficiency standpoint).
Put another way, a helicopter like the Mi-26 could probably achieve a fair bit more lifting power for its installed horsepower with a 2 blade rotor, but each blade would seemingly extend off into the next township.
when does having more rotor blades become less efficient?-- any number more than one. The problem is if you need more lift then you need to use longer blades but if you cannot increase blade length due to structural issues and/or storage issues then you have no choice but to sacrifice efficiency for more lifting power. A single blade is ALWAYS the most efficient number of blades. $\endgroup$