What's the chances of finding a stronghold without an eye of ender in Minecraft?

So I was mining in a cave near my house, and I found a wall of stone bricks and mossy stone bricks, and when I mined through, I was in a stronghold. So I was wondering what are the chances of this happening?

I only ever found 1 when I played.

There are more than 1 in the world, the chances are slim but it does happen. It has happened to me a few times