There seems to be a lot of dispute in the PPC community about how many text ads per ad group are optimal for split-testing. I’ve heard estimates anywhere from two to five ads. However, a lot of people don’t take into account the individual needs of their campaigns when selecting how many text ads they should test.
Personally, I’ve always been a fan of the three ad per ad group model. That way, you can rotate your ads evenly, and try out three different calls-to-action, value propositions, or other factors (dynamic keyword insertion, altered display URLs, etc.), and start collecting stats on which strategy works best. Three ads is a pretty good starting point for most accounts, but it doesn’t work every time.
What if you’re dealing with a campaign with a narrowly focused geotarget (i.e. one metro area), or a set of keywords that are highly specialized and don’t draw a lot of traffic? What if you’re dealing with both of these at the same time? You could be waiting for years before you get a significant enough set of data to determine which ad strategy works best. If you’ve been testing ads for several months and only getting a handful of clicks, it may be best to reduce your ad rotation to two per group just so that you can get significant comparisons sometime in your lifetime. The more ads you run per group, the less data you will get for each ad. It might not be a problem for a popular e-commerce product with hundreds of clicks per day, but if you’re dealing with a more specialized audience, two ads may be a better way to go.