Is "Disney travel agents" a specific company and you're asking about their specific policy? If not, then no, you can book as many cruises as you like and receive travel agent perks for each one, as far as
Disney cruise line is concerned. For example, Costco Travel provides a Costco Cash Card for 10% of the base cost of each cruise they either book for you or that you transfer to them within 30 days of booking. And that is the case no matter how many Disney cruises you take in one year. You get the same benefits for each one.