In Ruby, there is a total of 412 PKMN slots being used by the game. The last PKMN is Chimecho, which is number 412. The number 413 PKMN is an Egg. After that, Numbers 414 to 440 are UnownB up to Unown? .
Now, for your question about adding more than 440 PKMN, yes, it is definitely possible. I have already done it months ago, and just recently put it in my current RD hack. But, there is one error that I cannot fix: the cries. Some of the new 441+ PKMN has either Bulbasaur's cry, or no cry at all.
Anyway, how I did it was simply repointing a lot of data and copy+pasting old data for the new ones. There are so many tables you need to repoint though. I'm going to give you a list maybe tomorrow since I'm about to leave the office.
Also, for the cry table which is 0x1FDE6A, I believe that is the hoenn cry auxiliary table. Unfortunately, I have no idea how many bytes each PKMN has, but I'm going to try and find out tomorrow.
See ya later~