PCI Merchant Level Requirements for VISA are stated as the following:
Level 1: Any merchant-regardless of acceptance channel-processing over 6,000,000 Visa transactions per year. Also, any merchant that Visa, at its sole discretion, determines should meet the Level 1 merchant requirements to minimize risk to the Visa system.
Level 2: Any merchant-regardless of acceptance channel-processing 1,000,000 to 6,000,000 Visa transactions per year.
Level 3: Any merchant processing 20,000 to 1,000,000 Visa e-commerce transactions per year.
Level 4: Any merchant processing fewer than 20,000 Visa e-commerce transactions per year, and all other merchants-regardless of acceptance channel-processing up to 1,000,000 Visa transactions per year.
The other payment brands (MasterCard, American Express, Discover Card, and JCB) also have their own requirements for merchants.