It depends on what these sites are selling. If one is high risk (gaming, adult), you should have two merchant accounts.
But if they are all pretty low risk, you can have one merchant aco**** (no matter what those sales people tell you). They just want to get those accounts.
For an example, I have a friend who sells floor machine (those big buffers, etc). Some companies consider this a rask because of the size of the machine but we did not. He contacted me the other week - wanted to start to sell his cleaning / office supplies online. I told him no problem - we just update his merchant application to reflect what he is selling. And on his new site he has it in the footer & on the payment page that www.abc.com
is owned by www.xyz.com.
And even more on the payment page - that the charges on the card will reflect www.xyz.com
As long as you meet a few basic requirements - it can be done.