I believe the srcset are generated by WordPress, but @weaver can confirm.
I am not following your remark on the featured images. On the page you have us, these are thumbnail, and therefore have no srcset, as they are two small too ever need smaller resolution.
The srcset is meant to provide alternate resolution for very large image, so that on smaller browser size, a lower more appropriate resolution can be used. If an image is already as small (or smaller) as the smallest expected browser size, no srcset would be generated.
So the number of srcset depends on the resolution of the image. Depending on how small or large you may have zero or one or two or more srcset