During your trial period, you can add one Amazon or one eBay account.
This helps make the process of setting up easier and reduces the chances of anything going wrong in your first few days while you're testing the system. By focusing on a single Amazon or eBay account, you will also have a better overview of the results on your sales, generated by RepricerExpress.
Once you have signed up for a paying account you can then add additional Amazon or eBay accounts to RepricerExpress.
You can see how to add another marketplace here.
With Amazon, you can only have one Amazon marketplace of the same locale (1 Amazon UK and 1 Amazon US is fine, 2 Amazon UK is not possible etc...)
With eBay, you can have a maximum of 3 eBay accounts, whatever the locale (1 eBay UK and 2 eBay US is fine, 3 eBay US is fine etc..).