Johonn, that number is a variable used in the formula to decide how many mobs can spawn.
The formula is:
value * chunk_count / 256.
- value is the spawn limit number from the bukkit.yml
- chunk_count is the number of chunks currently loaded across all worlds
Another piece of information to know is that, by default on a SMP server, there is a 10 x 10 chunk square loaded around each player (you can modify this by changing the view distance in the spigot.yml or, if you're not running spigot, in the server.properties file).
So, with all of this in mind:
If you're alone on the server and in The End (If you're in the Overworld, spawn chunks would also be loaded but they're not since you're in The End.) that means there are 100 chunks loaded. So, plug that in to the formula along with the 70 from the bukkit.yml file and you get:
70 * 100 / 256
which equals, drum roll please....
That number look familiar? Bump your spawn limit up to about 250 and you'll get ~70 mobs all by your lonesome. :)
Edit: One last edit, as I was playing around with this on my own Spigot server. Given the case of an Enderman farm in The End, you can also increase gains by increasing the view distance for the server. Instead of 10, you can make it 12, or even max it at 15. This has pretty significant gains on spawn count for minimal impact considering you're in The End where there's basically nothing to render. You will be able to see back to the main island, but only faintly and it's still too far away for mobs to spawn.
Considering a monster limit of 70 and a view distance of 10 will get you ~27 Endermen, changing the view distance to 12 will get you
70 * 144 / 256, or ~39 Endermen. Maxing the view distance at 15 will get you
70 * 225 / 256, or ~61 Endermen.
Of course, if you have a lot of people on your server, this can significantly drag down performance because of all the chunk loading it has to do (unless everyone is close together and it can load the same chunks for everyone).