How many RAM can i add and who are the manufacturer

A hosed slot (other than the obvious visual short circuit if the metal connectors are in contact with each other) will possibly cause the machine not to start.

It can as well damage the memory stick (short circuit) when the computer is turned on with a stick in it.

If the voltage mess is big enough, in theory it could damage more than that (i.e the board).

Electricity in electronics is very delicate and if the wrong current or voltage flows to the wrong place, bad things happen.

Or… maybe nothing happens. Use a magnifier to see if the pins are aligned. If you see some in contact, it’s best to remove the battery and unplug the laptop and try to separate the pins that are in contact, even if you have to sacrifice the slot forever.

Repairing that is probably expensive (800++ u$) because you need to change all the mother board.

The System Information app has this information (and a lot of other info about your computer). Access it by going to the Apple menu (top left of your screen), selecting About This Mac, and clicking the More Info button.

Fire up the app and click on the Memory tab. You'll get a visual representation of how much memory you have and how it's installed. There is a rectangle for each memory slot, and each rectangle will be labeled "Empty", or with the capacity of the chip inserted.

For even more info, go to File-> System Report.
In the new window that appears, click Memory in the list on the left. It will show you the number of slots and what's in each one. In the below screenshot, there is one empty bank. It's the one with the Size, Type, Speed, and Status as Empty.
If none of yours say empty, you've got something in every slot.

Note that if you have an older version of OS X than Lion, you won't have the visual representation. You can open System Profiler (/Applications/Utilities) and follow the directions from the second half of this answer.