Minecraft – Mob grinder does not spawn more than 63 mobs

There are two points to remember that may seem contradictory: first, like Nicholas pointed out, you need to makes sure that your mobspawner is the only nonlit space in a pretty big area around you (I think the area's actually about 150x150 blocks), but second, you need a lot of empty space around. In other words, minecraft will choose places that have a lot of surface area (not volume) to spawn mobs in, so if you were to have a lot of open caves beneath your mobspawner, the game will flag that area as more likely to spawn mobs in. You just also have to make sure that it's all well-lit, then you've effectively tricked the game into flagging that area to spawn mobs in, but then conveniently your mobspawner is the only dark area where it can do so.

I would recommend you take a look at this tutorial. You may not want that particular style of mobspawner, but it goes over the prerequisites of any good mobspawner quite nicely.

Mobs won't spawn closer than 24 blocks. They won't move around when farther than 32 blocks. Those are the only rules you need to know for a grinder that relies on mobs walking around.

The inference is straightforward: you want your collection spot to be 24 metres away from the nearest wall of the grinder, and you want the far wall of the grinder to be 33 blocks away.

Or put another way, you want your grinder to be in the 8-metre-thick ring or shell around the player where mobs will spawn and move. In this illustration of the spawning distance rules, this thin shell of viable spawning area is represented by the light blue ring:

This implies that the optimal grinder is shaped like a ring or a ball surrounding the player. (Note that these distances are true radii, so if the grinder is above you in need to be closer than 24 horizontal metres!) Construction considerations make such a design less than optimal though, so a nice compromise is one to four towers that each fully enclose a section of the useful spawning shell; even if some of the floor space is "dead" sometimes, the less-perfect design allows you to move around in your collection zone a bit more and still have the grinder(s) work well.