Correct. Though it should be said that this jutsu can only be used with portals created with the technique, meaning though you can make countless portals in freeform, you can only use one portal with this jutsu. Though, say you release phoenix fire jutsu. The multiple bursts of fire would each contain bursts of Keigoku, ranging from as small as a tennis ball, to as a large as a beach ball, depending on how large the projectile used is. This can only be used with projectiles that contain your chakra however, so no kunais/shuriken with this.

That being said, if a jutsu contains chakra and is created as a projectile, traveling from you to the target, it can be used to hide Keigoku inside it. This jutsu lasts as long as you need it to, without the need to post the jutsu again, nor does it count as a jutsu per turn to hide Keigoku inside it. However, the portal cannot move and will remain stationary, so if you change location, you will have to use it once more.