It depends on what you want out of your legends. Sometimes the legend is so important for your deck that you're willing to risk drawing multiples of them. Other times the legend drawback is real, and you don't run four copies for exactly this reason. Some examples from recent Standard:
Adrian Sullivan's Jeskai Control won GP Milwaukee. Note the 4x Niv-Mizzet, Parun. This is a legend, but he has four copies. Why? Lots of factors go into this.
- Niv-Mizzet is a fantastic win condition: if you ever untap with him, you've more or less won the game (he's even more powerful than Teferi in this regard).
- Because Niv-Mizzet is so good at winning the game, Adrian Sullivan is dedicating other cards to supporting him - note he has Dive Down to protect Niv-Mizzet from targeted removal, and Treasure Map to accelerate Niv-Mizzet onto the battlefield. These cards (especially Dive Down) are not nearly as good if the deck doesn't draw Niv-Mizzet.
- Another way of thinking about this is that, this deck wants to draw Niv-Mizzet every single game, and the obvious way to maximize that chance is to run four copies.
Ari Lax's Selesnya tokens from Pro Tour Guilds of Ravnica. Note he has only three copies of Emmara, Soul of the Accord. Why? Again, many factors go into this:
- Emmara is a great role-player and a great body to Convoke with, but you don't need to draw Emmara. The turn after you play Emmara, you're still only convoking for one mana. If you didn't draw Emmara, you might have drawn another of the deck's 2-drops, which also convoke for the same amount. All your lose out on then is a 1/1 token - still a loss, but not a devastating one. Meanwhile if you draw Emmara without drawing a convoke card, she's not particularly good - just a 2-mana 2/2, easily prevented from attacking.
- Emmara is easily answered. Shock kill her at mana advantage (Shock costs 1 mana, Emmara costs 2). She's also fully vulnerable to all the commonly-played sweepers, from Ritual of Soot to Deafening Clarion.
- Emmara is not nearly as threatening as Niv-Mizzet. Niv-Mizzet is a removal magnet: it's fine if you can't play any more Niv-Mizzets, since because if you stick a copy of him, you win. With Emmara, if she lives, the game still goes on. In this case any more copies of Emmara in hand become genuine dead cards.
If you followed this, it should be clear why there're only three copies of Emmara in the Selesnya deck. The first copy of Emmara is great, but the second is not. But for the Jeskai deck, casting and protecting Niv-Mizzet is its gameplan. It wants to draw a Niv-Mizzet every game. It doesn't want to draw multiples, but if the first copy dies, it immediately wants to draw another. So it runs four copies of the legend.
Here's a heuristic to decide how many copies of a card you should run.
- 4: I want to draw one copy every single game. The card is so important I don't mind drawing multiples (or in the case of some cards like History of Benalia, drawing multiples is actively a good thing).
- 3: I want to draw one copy every single game, but I don't want to draw multiples.
- 2: I'm indifferent about whether I want to draw this. If I draw it, fine; if not, that's also fine.
- 1: I don't particularly want to draw this, but if I do, it can have some niche effect that's strong enough for me to include it in my deck anyway (or I have effects that tutor for this card).
On a side note: it sounds like you're including planeswalkers because they're "cool". There's nothing wrong with that, but if your aim is to make a strong deck aimed at winning, each card must have a reason for being included. Take Huatli, Radiant Champion. What does this card do for you? Does it make cats? Does it synergize with cats? Is it better than simply casting another creature like Brimaz? I'm pretty sure the answer to all these questions is "no", and so you should not be including her. There's a place for planeswalkers like Huatli, but in a deck like yours, that place is the sideboard. If you're playing against a deck with lots of Cleansing Nova and Golden Demise, then you bring her in because she's a threat that survives these sweepers. But in game 1, when many decks don't have sweepers, you're better off just casting another creature. Among the planeswalkers you listed, the only one I can see playing in the maindeck is Ajani.