In the meantime (εν οις). It is a classic idiom to start a sentence or even a paragraph as here with a relative, "in which things or circumstances," without any expressed antecedent other than the incidents in Luke 11,53Lukas 12,3

Many thousands (μυριαδων). Genitive absolute with επισυναχθεισων (first aorist passive participle feminine plural because of μυριαδων), a double compound late verb, επισυναγω, to gather together unto. The word "myriads" is probably hyperbolical as in Acts 21,20Apostelgeschichte 19,19

Insomuch that they trode one upon another (ωστε καταπατειν αλληλους). The imagination must complete the picture of this jam.

Unto his disciples first of all (προς τους μαθητας αυτου πρωτον). This long discourse in Luke 12:12Luke 12:12

Beware of (προσεχετε εαυτοις απο). Put your mind (νουν understood) for yourselves (dative) and avoid (απο with the ablative).

The leaven of the Pharisees which is hypocrisy (της ζυμης ητις εστιν υποχρισις των Φαρισαιων). In Mark 8:15 Jesus had coupled the lesson of the Pharisees with that of Herod, in Matthew 16:6 with that of the Sadducees also. He had long ago called the Pharisees hypocrites (Matthew 6:2; Matthew 6:5; Matthew 6:16). The occasion was ripe here for this crisp saying. In Matthew 13:33 leaven does not have an evil sense as here, which see. See Matthew 23:13 for hypocrites. Hypocrisy was the leading Pharisaic vice (Bruce) and was a mark of sanctity to hide an evil heart.