I would probably keep like 7-8 in there but I bet you could do 10 without any problems if there is a bunch of food. Of course you would have to seperate as they grow or move them to a larger enclosure. You do not want to keep different sized mantids together either, the bigger ones will eat the smaller ones.
I have four (3 male one female) L4 or L5 ghosts (all on same instar though) in an exoterra nano tall (12x12x18) and they're doing just fine, though I plan to separate the female as soon as they start to molt.
That will definatley be fine for 10 for a while. Make sure there are plenty of different places to perch, also I like to hang strips of mesh or straw or even string so that there are mupltiple ways up or down from each branch (that way they are not forced to confront each other when trying to move to a different perch).