Hey there all.
I have 9 rattie boys.
They are currently living in a very large, custom made ferret cage.
I HATE HATE HATE this cage.
It's a pain to clean, smells too easily, I feel like there isn't enough room, even though it's 1.5X the FN, it's just set up weird. And I have had it for a couple years now.
Now, some of my rattie boys are quite...chunky. Two handers. Lol.
Only a few of the younger boys (About 6 months now.) are smaller. And they'd habve a helluva time getting out of the FN. They aren't small.
I was planning on adopting another boy around Christmas/New Year's time, or chortly after (There always seems to be an abundance of unwanted rats after Christmas.)
Would the FN be big enough for ten hefty boys?
I know it says 12, but I want them to have breathing room.
I'm asking for the FN 142 for Christmas. And I've already began making liners and such. If I have to I'll save up more for the third tier, but I'd prefer not to. Some of my boys are fast approaching 2+, and I like to have as much money as possible in my vet fund.
Any opinions?

Personally, I wouldn't put 10 in one.  I think I'd probably max out at 8.  


Another option, since you said they were getting older...  Instead of adding a third level in height, what about modifying it so it's 1.5 FNs?  A 141 and a 142 side by side.  This way, you'd have plenty of space for all of them, but you would also have more floor space if anybody starts having issues with hind end degeneration.  Then if they can't do ramps as easily, they still have plenty of room to move around.

This is what I hope to do with ours some day, although if there isn't a huge price difference, I'd probably spring for two of the 142s.  Another reason I vote against adding a level up is I'm short...  5'2".  Without a stepstool it'd be hard for me to clean the top section.