Second example. Say you are in the US and you do all your processing via Auth.Net. That's one token. Now you open a local office in Australia and want/need to process locally (say PIn Payments) That's a second token. Now you expand to the UK and use Sage Pay there (third token)
Final example. You're building the next Shopify or Freshbooks. Each of your customers will do their own processing and need their own merchant account. Each of those customers equates to one payment gateway token.
So a stand alone commmerce site with one gateway for credit cards and support for Dwolla and PayPal as a payment type = 3 tokens. The Uber model - where in each country you set up a new processor - probably equals 4 - 10 tokens. And then next Shopify = 100's to 1000's of tokens.