Using a filter for more than one OCI depends on how clean the engine currently is, the total planned miles put on the filter, if you plan on using something that will start cleaning any deposits (like HPL oils or the new Valvoline Restore & Protect), and of course what filter you plan on using. As mentioned, you don't really want to use a low efficiency filter for long total use, because if it loads up it will slough off already captured debris worse than a high ISO rated efficiency filter. Use a high efficiency filter with a long rated use - The Endurance and a few others fit that requirement.
I'd do what you originally thought ... I wouldn't push the Endurance, or any other filter, to it's full "up to" use rating unless the engine was pretty new and spotless inside. If you were doing 6K-7K OCIs and the engine was clean inside, then going 18K-21K on the Endurance would be a decent use out of it. If doing 8K-10K OCIs, I'd just use the Endurance for two OCIs ... give it some headroom.