There are 6 15-letter words that start with 'ven'.
| venerablenesses | ventriloquially | 
| ventriloquistic | ventriloquizing | 
| venturesomeness | venturousnesses | 
Note: These 6 15-letter words that start with 'ven' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with vent, vene, vend, veno, veni, vena, veng, venu.
4-letter words start with 'ven' (3)