How many are they? I heard somewhere there are 16 (one of the hints in the game said there are 16 daedra princes each with their own plane).

I have found and closed 14 to date (including Kvatch, the initial Bruma gate and some of the city ones). Some were easy to find as they are near cities and part of the quest but some I just found by accident scattered around. My concern is that I have to go through the whole map again hoping to bump into one.

So just for my peace of mind how many are there excluding the ones that will open in the big fignt for Bruma (I havent gotten there yet)? Or will they keep opening randomly until I do the main quest? There is cool stuff in them which are missing from my collection so if the will keep appearing I may need to postpone my main quest ending.

Thanks

There are 50 random gates that can appear around Cyrodiil, 9 standard gates, and then the Great Gate at Bruma, for a total of 60 gates.

However, there are only so many that can spawn during certain times of the main quest. You have to have completed the "Dagon Shrine" quest in order to have the maximum 50 being capable of spawning. The cap will remain at 50 until after you leave Paradise.

The plane of Oblivion you are entering is Mehrunes Dagon's Deadlands every time. The other Daedric Prince's realms are very different. If you have the Shivering Isles expansion, you'd see just how different Sheogorath's Madhouse is by comparison. It's actually liveable. Sort of.