Greetings @Azulix
I appreciate and thank you for trying to help them @I3dontknow32.
Are you trying to imply the Party EXP Share?
In case you are, Party EXP Share was disabled with the end of Halloween event on 13th of November. Thus, you can no longer receive EXP points on all six of the pokemon in your party by defeating pokemon with one.
You can, however, use the item EXP Share, making a pokemon hold it you can share the EXP to one additional pokemon at once. Thus, battling a pokemon in wild/NPC battle will now result into sharing the EXP between two pokemon at once. In case you haven't, you can obtain this item in the following places from the Professor's Aide NPC :-
Route 2 Stophouse 3, can be reached from going into the Diglett's Cave from Route 11.
National Park Stop House 1
Mauville Stop House 3
Route 214 Stop House
As they mentioned above, EVs max out at 510 total and 252 for each stat. So, it cannot gain any further EVs after it is maxed out.
I hope this helps answer your question. Feel free let us know if there are any other concerns regarding this.
Have a great day,
Fus1onflare