Ambitious Dragonborn enters with X +1/+1 counters on it, where X is the greatest power among creatures you control and creature cards in your graveyard.
Ambitious Dragonborn is not yet on the battlefield when its effect looks for the creature or creature card with the greatest power, so any creatures that have a bonus to their power based on the number of creatures on the battlefield won't count it until after it enters with counters.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.