Adam Clarke on Luke 12:1

An innumerable multitude of people - Των μυριαδων του οχλου, myriads of people. A myriad is ten thousand, and myriads must, at the very lowest, mean twenty thousand. But the word is often used to signify a crowd or multitude which cannot be readily numbered. There was doubtless a vast crowd assembled on this occasion, and many of them were deeply instructed by the very important discourse which our Lord delivered.

Leaven of the Pharisees - See Mat 16:1-12.

Which is hypocrisy - These words are supposed by some to be an addition to the text, because it does not appear that it is their hypocrisy which Christ alludes to, but their false doctrines. They had, however, a large proportion of both.