You can have unlimited planeswalkers on the field at once, but only one of each. So, for example, you could have Ajani Goldmane, Jace Beleren, Nicol Bolas, Sharkhan Vol, and Elspeth Knight Errant on the field at the same time, but the planeswalker rule states that if at any two planeswalkers on the field at the same time share subtypes, they both go to the graveyard. (Like the legends rule) For example, if you have an Elspeth out and you or your opponent plays another Elspeth, they both die.

Be careful with Chandra Nalaar and Chandra Ablaze, and Ajani Goldmane and Ajani Vengeant. The two chandras both share the subtype "Chandra" and the two Ajanis share the subtype "Ajani". If you have Ajani Goldmane and Ajani Vengeant out at the same time, they both die. (You can use that rule to kill an opponent's planeswalker)

306.4. If two or more planeswalkers that share a planeswalker type are on the battlefield, all are put into their owners' graveyards as a state-based action. This is called the "planeswalker uniqueness rule."

[[Chandra Nalaar]] [[Chandra Ablaze]]

[[Ajani Goldmane]] [[Ajani Vengeant]]