EntityPlayer:GetNumFamiliars()

[Foks256]

Returns the current amount of said familiar, would be something like this piece of code:

function mod.GetExpectedFamiliarNum(player, collectible)
	local pEffects = player:GetEffects()
	
	return player:GetCollectibleNum(collectible) + pEffects:GetCollectibleEffectNum(collectible)
end