PC 'You and what army?' achievement can be earnt even when your max summons is just 8.

[Invisisolation]

Steam or GOG

Steam

Single Player/Multiplayer

Single

Operating System

Windows 10

Terraria Version

1.4.1.2

Controls Used

Keyboard/Mouse

I'm currently playing an Expert Summoner run, my current summoner gear includes the Sanguine Staff, Tiki Armor, a Papyrus Scarab, a Pygmy necklace, and the Bewitched buff, which should add up to just 8 Sanguine Bats. When I tried to summon a 9th minion, the game despawns one of them but gave me the achievement anyway.

 

[Peanut sauce]

You may have been at the mob cap.

 

[Dapling]

Did you get the Summoning buff from the potion and forget about it?

You may have been at the mob cap.

Summons use the projectile cap, not the mob cap. You basically can't hit the projectile cap unless you're utterly spamming something like a water bolt for ages.