How many Field Spell Cards can you have in a deck?

You can have as many Field Spell Cards as you want in a deck, as long as it follows the general deck construction rules:

* Minimum 40 cards: Your deck must have at least 40 cards.

* Maximum 60 cards: Your deck cannot exceed 60 cards.

* No more than 3 copies of the same card: You can only have a maximum of 3 copies of the same card, except for basic monsters and some specific cards.

So, theoretically, you could have a deck filled with 60 Field Spell Cards. However, it is highly unlikely that such a deck would be effective in actual gameplay.