What are nether fortesses and how can I find them?

How to find Fortress in Nether world?

They're there, and will spawn in every world (assuming you aren't using a really old version of Minecraft), though, they can sometimes be difficult to find.

There are a few "quirks" in the generation code that you can take advantage of in order to make finding the nether fortresses easier. Tips taken from The Minecraft Wiki on Nether Fortresses:

• Nether Fortresses tend to spawn in "strips" aligning along the Z axis (north/south) in the nether. (Tested over 200 seeds so far, the trend is strong in all tested seeds - confirmed with AMIDST.)
• If you don't see a Nether Fortress within 100 blocks or so north/south of your starting position, it's best to search diagonally or east/west. If you continue searching north/south, you may be between strips and never find one.
• On the flip side, if you've found a Nether Fortress, it's best to search north/south from there to find more.
• When trying to spot nether fortresses, a potion of nightvision can help considerably.

If you are not opposed to using external tools or mods, there are several tools which predict where structures will be generated, and create a map for you. The only one I know of off the top of my head (although there are several out there) is Amidst which creates a map of your specific seed, and should work in the nether as well. This will help you find the structures without having to create those chunks by exploring the area first.

Is Nether world almost limitless like overworld or limited size?

The Nether is exactly as limitless as the overworld! (which means, there is technically a limit several million blocks out at which point it starts overwriting old chunks, but there is no chance you can reach that limit "accidentally", you need to travel in a straight line for several real-life days without stopping.)

Is there only 1 Fortress in Nether world or many?

There are many! While there is a hard limit of three Strongholds per world, there is no limit to the number of Nether Fortresses; you can keep on traveling farther and farther, and new fortresses will keep springing up along the way.

If you're trying to find a nether fortress, go to settings, and then Video Settings. Just turn up the brightness all of the way and the render chunks some. Not all the way, because it can cause lots of lag for even fast computers because of the "Deepness" of the Nether. It goes along way, and its just a lot for computers to render. Mine works at about twenty, but for slower computers I'd only recommend around 15. Potions can't be used because a brewing stand required blaze rods to make, and blazed spawn in Nether Fortresses Only.