Rashi on Exodus 1:7/רש"י על שמות א׳:ז׳

2וישרצו AND THEY INCREASED ABUNDANTLY (more lit., they swarmed, a word used of the prolificness of reptiles) — they bore six children at one birth (the six words from פרו to מאד suggest or imply the number six) (Midrash Tanchuma, Shemot 5).