How to get loads of zombies on minecraft?

Change difficulty to hard, wait for night, go out and make sure there's no torches around, also you can go dungeons and caves.

Just build I giant mansion with lots of dark spot. I'll tell ya. It's creepy. Every night my mansion will fill up with zombies, spiders, ended man, those stupid skeletons. All those mobs. It really has a haunted mansion vibe going on in it. I made it in survival mode. Not creative. So it took ages to build. But it's by far the scariest thing I've built in minecraft.

You can try this glitch where all you do, make sure the sun is aligned. Put two stones down put a third stone on top of the 2nd stone placed down. Then pit an item frame on the side of the 3rd stone that placed on the 2nd stone. Then place a Redstone Comparator behind the stone you put the item from on. Now, place a zombie spawn egg in the item frame, wait til night time ro spawn the zombie in front of the item frame, once you do that exit out of minecraft then come back to your minecraft server and there should be Loads of zombies in your world by now.