With Arachnoid Horror can you play multiple units to the battlefield?

Yes, you can play multiple units to a battlefield using the ability granted by Arachnoid Horror.

According to the card text, Arachnoid Horror provides the following ability:

"Friendly units can be played to an occupied battlefield if an enemy unit is alone there."

This is a continuous effect that modifies the rules for playing your units. As long as the condition is met (an enemy unit is alone at that battlefield), you may play multiple friendly units there directly.

Important Considerations:

• Condition: The ability specifically requires that an enemy unit is alone at that occupied battlefield (i.e., there are no other enemy units at that location, per Rule 741.1).
• Exhaustion: Units played directly to a battlefield—whether via this ability or standard play to a battlefield you already control—enter the battlefield exhausted (RiftJudge FAQ #7438).
• Battlefield Capacity: While there is no hard limit on the number of units that can be at a battlefield, remember that you cannot play a unit to a battlefield that already has units from two other players present (Rule 444.2), and if you are playing in a multiplayer format (3+ players), you must adhere to restrictions regarding battlefields with ongoing combat (Rule 457.2).