Having an optimum opening hand is so crucial to performing well in a game of Magic. The more lands you have, the fewer options you have when it comes to spells to play and on the other hand (haha, get it?), fewer lands means you may not be able to cast your stronger spells at the optimum time. So what is a good land to spells ratio for an opening hand? Honestly, that’s going to depend on your deck and the rest of the cards you draw in that hand. A red deck that seeks to win fast with cheap spells is going to want fewer lands than a blue control deck that runs a lot of counter spells, but as a rule of thumb, most decks want to see 2 or 3 lands in their opening hand.
Lands alone shouldn’t be the deciding factor in weather to keep a hand or take a mulligan. The cost of your spells plays a huge role as well. The player that usually wins is the one who can spend their mana in the most efficient way, which means playing the right spell at the right time. Say that you’re playing a green deck in the current standard environment (War of the Spark at the time of this post), and your opening hand contains two basic forests, a Llanowar Elves, a Steel Leaf Champion, a Carnage Tyrant, a Giant Growth, and a Vivian Reid. You obviously wont be able to play Vivien or the Carnage Tyrant anytime soon, but having that Steel Leaf Champion on turn 2 would be great, unfortunately that means you wont be able to play Giant Growth which could keep the Steel Leaf Champion alive, especially against a deck that uses damage based removal, like Lava Coil from a mono red deck, which is everywhere.