Can Hidden cards be played to base or just battlefield, and is there a limit to the amount of Hidden cards that can be on the field?

Ruling: Hidden cards can only be placed at battlefields, not at base. Only 1 card can be Hidden at a time unless you have the Bandle City battlefield.

Nuances:

• You can play the card normally without hiding it, which removes the placement restriction
• If Hidden cards somehow get to base, they will recall