The DOT limit is 60.

I highly doubt that all 24 people are using enough DOTs to reach that limit.

I use SMN in Void Ark and even though sometimes I can't see the duration of the buff, it's still active since when I use Fester it does full damage.

You'll know if a dot doesn't apply because the floating text will say "no effect". If you don't see that, then the dot is applied even if the display doesn't show it