There is no limit on how many frames or weapons you can have, but you will have to spend plat to get more room or sell old frames and weapons.
You can also have multiple of the same, but you only get Mastery Rank xp the first time you level a frame or weapon. Having multiple of the same weapon will not give you more Mastery Rank xp.
Currently there are 16 frames (14 normal and 2 prime) and about 89 weapons in game.
If you level them all you will reach rank 12 and be 6000 points in to the rank.
So in other words to reach rank 12 you either need all weapons and 15 frames or all frames and miss out on 2 weapons.
There are currently 4 weapons that are no longer available in game so you need to own 2 of those weapons to reach rank 12.
The weapons you can no longer get are Lato Vandal, Braton Vandal, Snipetron Vandal and Snipetron.