Usage: 3 forms in 3 verses
Form Translated As     In Verse(s)
πολυπληθείτε you were numerous LXX.Deut.7.7
πολυπληθεί is numerous LXX.Lev.11.42
πολυπλήθει are numerous LXX.Exod.5.5

Liddell-Scott-Jones

πολυπληθέω (poluplētheō) be numerous G:V

πολυπληθέω-έω,

multiply, [LXX]; π. ποσί abound in feet,[LXX]

LSJ Scripture Refs: LXX.Exod.5.5LXX.Lev.11.42