There's a thought that having more cards on the battlefield leads to better outcomes. And there's some truth to that. For one, you get the summoner boost for more cards. That has a multiplier effect as you get more attacks and your opponent has to take out more cards. But sometimes, it can be better to have fewer, but stronger cards.
When to concentrate your power
Here's a battle that shows the difference. My opponent had 6 cards while I had only 4. Then, my opponent also had quite a big summoner boost with 4 cards getting a boost while only 2 of mine got one. So at first glance, it isn't looking good for me.
But by the middle of round 2, it had evened itself out. Still, my opponent had a decent attack advantage.
And then it comes down to the final two. It was going to be a slug-fest and that's where having a proper tank like the Imperial Knight made a big difference because of its healing ability.
Eventually, I won the battle even though at the start, I think most would have been doubtful looking at the lineups. You can catch a replay of the battle here: https://splinterlands.com/battle/sl_f8342515563b7a1ecc23b192de8cc682?ref=charcoalbuffet
Quantity vs Quality
It's the age old debate because there are mana constraints - quantity or quality. My personal preference is to choose as many heavy hitters / tanks as I can. The only exception is if the summoner boost is able to make an outsized gain. That was the case with Alric but since playing in Modern, I have switched to fewer but stronger cards.