You start the Pokedex with a number of pokemon, you can then recieve up to 3 pokemon each day. This will stop when you have one of each of the pokemon. From then on in order to gain the other forms (I.e summer deerling or pansear) you need other people with the Pokedex to give you them. Some of the legendarys can only be recived by scanning their AR markers... Hope this explains everything.
P.S. I Have a full pokedex