There is a difference between the vet teleport tiles and the house teleport tiles from house customization.
Yes, if I'm not mistaken, you have only the lockdown limit of the castle/keep for the amount of teleporter tiles you can lock down in your castle/keep.
Customized houses up to 18*18 however are limited to only 12 teleporter tiles plus the allowed number of lockdowns for teleporter tiles.
It's highly unlikely anyone would want to put so many house teleporter tiles in, I think just having one for the central roof and four corners should be enough for just about anybody. Maybe one or two extra to get around the sides faster, but no more than 10 teleporter tiles should be needed for a castle.
Perhaps players would build a teleporter library for houses on a shard, but I think a rune library would be better and easier to maintain.