Congrats on receiving an offer. You counter when you think the offer is inadequate or you know they are lowballing. There is no such rule as you shall counter at least once. Don’t get hung up on that idea.
And when you feel you need to ask for more, never set a target like 5%, 10% or specific amount. It is like declaring that I am out if this condition is not met. Simply politely ask if they can do a bit better. Of course, there are exceptions. If you already have a better offer from other places, you can be more specific about your demand.