Statistically, 30 to 50 responses will give you a fairly accurate count on what works. When you get 100 responses, you’ll have an accuracy count of almost 95%. Stunning, isn’t it?
You can handle this: either split your list in two randomly or separate a smaller list to test which element does better, before sending to the full list. Now, how big of a list do you need? That depends on your click through averages.
For example: your list contains 10,000 contacts. Your average click through rate is 8%. Consider how many responses you need (let’s say 100). With an 8% click through rate, sending out 1300 emails would result in approximately 104 responses. That would give you a very accurate count for your test. If your click through rate is higher, the number of emails you need to send in order to test would be lower. And obviously, the larger the response you get, the more accurate your results will be.
Conclusion: always consider the response rate you need in order to get accurate test results.