When we are talking about Splinterlands Battle, we will given chance to put lane up with some limit, normally it would be 1 summoner plus 6 monsters, but sometimes, it could be just 5 or 4 for certain ruleset.
Most people would try to fill all the slots because normally people would think with more monsters, the more they can do on the battlefield, and the higher of winning chance we can create. While at some situation it's true, sometimes it doesn't ! Let's break it down !

#1. Not all buff or ability affect all monsters

If you are veteran, then you knew that there are some buff from summoner that are applied for all monsters while some others are not, especially at the newest series at modern league where summoner have optional buff, and most of it are limited into certain amount of monsters. If the buff are applied for all monsters, then it's make sense that you try to get the most of it by getting as much monsters as you can, but if it only given to 1 or 2 monsters, why doesn't max it out by having less monsters but could giving us bigger impact ?

#2. Ability or Ruleset that Affected by Eliminated Monsters

This is the second reason why it's important to considering your monster selection. Some ability can heavily affected by how many monsters are eliminated. First example is the scavenger, which will give the monster who had the ability 1 health point everytime there is any monster eliminated. The second example is born again ruleset where all monsters can reborn. At this ruleset, more monsters mean more reincarnated and more attack needed by enemies to eliminate your team. Then there is the bloodlust where the monsters can bulked by eliminate enemy monsters.

You need to carefully thinking about the ruleset and the possibility of your enemy line up. If you think your enemy would have bloodlust monsters or scavenger ability, don't throw some mini monsters that they can easily eliminate to give more advantage for your enemy.

#3. Remember the Main Mission

This is the basic that sometimes are forgotten. Splinterlands is about eliminating your enemy first or try to survive longer on the battlefield. So having less monters or more monsters is not the most important thing. If you can eliminate your enemy faster with less but powerfull monsters, then go for it !

The Battle Example !

Okay, now let's take a look at the battle to prove my theory is right. You can watch the full reply at https://splinterlands.com/battle/sl_dae65246d2279550013953a9c30e4aa8 , but I will also share it here.

Here is the formation and rulesets of the match. The monsters option are heavily reduced with melee only ruleset is coming there because you can't deploy monsters that doesn't have melee attack. The good news is you have reach ruleset to support the melee monsters to be able to attack from second position. Then there is the stampede ruleset which giving advantage for monsters with Trample. Mana cap for this match is 36, it's medium mana given here.

Let's see the formation. My enemy looks scary with max level summoner and max level monsters on his side. It's very complete with all monsters able to attack from the back, and there is inspire to support it. Some also have range and magic damage which makes their attack even more dangerous !

At the other side, I only have 2nd level of Legendary summoner and deploy 4 monsters only. The reason is because I spent a lot of mana for the summoner and I want to max out it's Armored Strike buff for a big monster with big mana cost, the Commander Slade. I also decide to have tanker with shield ability because it's melee battle (even that there is still magic damage, it's fine because the skink is having the phase ability !). On the back, I try to give another monster with shield so it can't survive longer by taking half melee damage. At second position, I got my main hitter, the Commander Slade, while at 3rd posotion there is another monster for survive with it's regen ability. I put him there to tank from the possibility of sneaking assasin, and if the Dino is eliminated, he can attack from second position with the reach ruleset.

How the Battle Goes

At first round, both theam are losing 1 monster. The backline of my team and the tanker of my opponent team. Still looks disadvantage for my team with 3-5 situation at the end of 1st round.

Now there are 2 monsters are eliminated on my opponent side, and meanwhile on my side it's only the tanker that eliminated. Now it's 2-3 situation at the end of 2nd round.

Both team are losing 1 monsters at 3rd round, so now I left with 1 monster and have to face 2 monsters on my enemy side.

But my monster can single handled both of the enemy monsters and win me the match !

How My Team Could Win ?

How can 4 monsters with a half of it's max level can beat 6 max leveled monsters ? This is because my main attacked, the commander slade is very effective to eliminate my enemy. The buff given from the summoner really help, and the commander slade stat's make the effect even bigger with armored strike is based by the armor of the monster. My commander Slade can easily eliminate enemy with the pierce and 7 damage of armored strike, something that can be hard to be done by low mana monsters. This attack itself can easily eliminate small monsters. Then it will do the main melee damage which also very good because it has high damage.

But not just with the main hitter, the other monsters really did great job to waste enemy attack with their surviving ability such as the shield and the regen. They give the Commander Slade enough time to eliminate all monsters on my enemy side !

Conclussion

On Splinterlands, the number of monsters is not everything ! The most important thing is trying to eliminate your enemy as effective as possible, and sometimes the big monsters can doing it better because they have better stats and ability, especially with the buff of summoners that can make them even more dangerous. You need to really considering the buff effect, mana cap, monster's ability and ruleset before decide to go with more monster or less monsters. At the very end, it's not who have most monster would win, but those who can eliminate the enemy lane up first !

Final Words