Is it too crowded to travel in a 6 berth couchette?
I traveled in one on a night train from Florence to Munich a few summers ago. When we left Florence at about 10:00pm only four of them were taken. We must have stopped somewhere along the way and picked up the other two because when I woke up at 6:30am in Munich they were all filled. I was asleep the whole time so didn't notice if it was crowded or not.
Compared to a 4-person couchette, there are 50% more people, if both are full.
On the other hand, there are exactly the same number of people per bunks as in a (full) private sleeper cabin. In other words, there is room for everyone. So it depends what you mean by "too crowded."
The upgrade to a 4-person couchette compartment is about half again the cost of the 6-person version. I think that is worth it because a full 6-person compartment has a 60% greater chance to include someone other than you who snores!