It's back! Zoya kicked off their Nail Polish Exchange on Earth Day, and this will last a whole 2 months! I still can't get into the site myself, but from what I have seen from others and from Zoya's Twitter [official Twitter page], there is NO MAXIMUM for this, but there is a minimum of 6 bottles per order. Additionally, you have to include $3.50 per bottle. You can send Zoya your old top coats, base coats, polish (even mini bottles!) but you can only order Zoya polish for your exchange.
From Zoya: Zoya Nail Polish will exchange your old, out-of-style, unwanted nail color and replenish your nail polish supply with fresh Zoya fashion colors. Simply send in six or more bottles of old polish - any brand other than Zoya, Qtica or Nocti, new or used - and we'll send you the new Zoya colors of your choice in a bottle-for-bottle exchange.
To start exchanging your polish, please click [here] to go to Zoya's website. *In case of crashing, please wait and try again.