On our first bareboat trip, 16 years ago, we had 10 of us on a 38' cat. Never seemed crowded. Last year we had 10 of us on a 50' cat, seemed a little crowded....LOL. It really depends on what you get use to, we keep moving up in size, so we are getting spoiled.

I think on a brand new 40' Lagoon, you will have plenty of room for 6. We prefer to have a head for each couple, but if that is not important, then you should be fine.