[Custom Feature Question] Item that shows the encounters in the area

I want to create an item that shows a certain number of encounters that occur in the area in which the player is located.

For example: if in Field encounters are;
Rattata, Pidgey, Purrloin, Pidove, Starly, Bidoof, Sewaddle, Ralts, Sentret, Igglybuff, Audino, Tauros
I want to show only to:
Rattata, Pidgey, Purrloin, Pidove, Starly, Bidoof.
Only the half of them, since the rest will be shown with another item.
Sorry for my bad English, I talk spanish