12 hours does seem low but some LOs are like this.
The most telling aspect of whether there is enough sleep is his mood. If he is generally happy and has energy then he is probably getting enough. If he is falling down, bumping himself more than usual (all babies bump around but I mean increased), grumpy, short tempered, and body language such as eye rubbing, yawning, then he might not be getting enough sleep.
There isn't really a BT that is too late as such. If your day ends up longer because you are fitting in two naps then BT will naturally go later. I know our BT went as late as 8pm for certain phases of routine changes and other people go even later. Yes the night gets short but because there are two naps in the day LO can be well rested. I would look for at least 10hrs at night.