According to Fessic's Beginner's Guide to Console Commands, which I found via Google expressly to answer this question, the maximum amount is 65,000. I haven't tested this myself, but it sounds like he knows what he's talking about:
Add Items Directly to Your Inventory: player -> additem "[object ID]"
I realize this one's a bit confusing so let's try a few examples:
Be sure to pay attention to the spaces before and after the arrow thingy.
Here's a few more Object IDs just to familiarize you with the naming convention:
- Sload Soap, "ingred_sload_soap_01"
- Quality Feather Potion, "p_feather_q"
- Lantern, “light_de_lantern_06_128”
Add Gold to Your Inventory: player -> additem gold_001
Example: player -> additem gold_001 535
Greedier types should note that the maximum amount you can add per execution is 65,000
This is probably a good time to mention that if you open the console and press the up arrow, your last command entry will come back up. Keep pressing up to scroll through all previously entered commands. Could save you a little typing now and then...
The last note on pressing the up arrow is pretty useful, definitely use that.
A Note on Binary Math
I would not be surprised if the actual number is 65535, which is the 16th power of 2 (minus one). This is the largest possible number that can be stored in a 16-bit unsigned integer.
To support this, note some math:
99999999999 (decimal) = 1011 1010 0100 0011 1011 0111 0011 1111 11111 (binary)
Truncating that to 16-bits gives
0111 0011 1111 11111 (binary) = 59391 (decimal)
Thus explaining why it has been giving you that particular amount.