how do you know the number of the subject ?

I need to know the number of the object , for example :

if($p->getInventory()->getItemInHand()->getId() === 1){
$p->sendMessage("hmm okay");
return false;
}

and I need to accomplish something if the stone at count 10

PHP:

$ev->getItemInHand()->getCount();


Tip: read THE DOCS

If you are trying to get the item of a hand of a player it should be:

PHP:

$player->getInventory()->getItemInHand()->getCount();


To learn more about how to do something with a player's inventory, look here: https://github.com/PocketMine/PocketMine-MP/blob/master/src/pocketmine/inventory/PlayerInventory.php

No, you should take your own advice and read the "docs". It's the "source code", not the docs. Please make sure you know what you're doing before replying.