So far my porygon is level 49 and has tackle tri attack zap cannon and psybeam hope i can help =).
there are 151 between leafgreen and fire red
150
first you have to get your national pokedex. then go to the Pokemon mansion. save the game in front of masters room, don't save it after!!! then go in and talk to him. if he does not say porygon turn off the ds and try again until he says porygon. then go into the trophy garden. eventually you will find porygon NOTE this wil only work for Pokemon diamond an pearl!!! if you have LEAFGREEN or FIRERED theres only one way to get porygon.first you go to the game corner in celadon city and buy as many coins you can get. the biulding to the right has the Pokemon porygon as a prize if you have 9999 coins you can also get Abra for 180 coins a clefairy for 500 coins a dratini for 2800 coins and a scyther for 5500 coins.AND YOU CAN ONLY GET THESE FROM THE GUY IN THE MIDDLE.
There are 2
Only 1.
In total including event only Pokemon there are 10.
Only one
Of the many Pokemon that you can't get in leafgreen are: Ekans, Poliwag, Scyther, Delibird, Oddish, Murkrow, Growlithe and many more. Edit: Poliwag can be found using a super rod in the pond at route 6.
To get a Porygon you will need to purchase it from the Celadon City Game Corner and it is sold for 9,999 coins so if you want to make things easy buy the coins from the cashier then use them to buy Porygon.
7, i believe.
There are 7 non-event islands.
You can get 2.