There are a total of 19 11-letter words containing 'plex', and the number may vary in different dictionaries. You can specify the dictionary and order.

• amplexicaul ₃₀
• cataplexies ₂₅
• complexions ₂₉
• complexness ₂₉
• metroplexes ₂₅
• multiplexed ₂₈
• multiplexer ₂₇
• multiplexes ₂₇
• multiplexor ₂₇
• overcomplex ₃₂
• perplexedly ₂₉
• uncomplexed ₃₁
• duplexities ₂₁
• complexones ₂₄
• complexuses ₂₄
• pleximeters ₂₂
• pleximetric ₂₄
• unperplexed ₂₃
• unperplexes ₂₂

There are 19 11-letter words containing plex in the well-known word game dictionary, 11 in the Scrabble US (US/Canada/Thailand - TWL/NWL) dictionary, and 18 in the Scrabble UK (International - SOWPODS/CSW) dictionary, and 12 in the Words with Friends (WWF) dictionary.