Can you confirm that you are running 2015.1? If not you will need to sign out of your CC account and back in again to force the update to appear.
I've merged 11+ images using the pano feature so there is no limit as far as I know: what draws you to the conclusion that it is ignoring the images? Are they at the ends of the image or in the middle?
If you were using a Trial version or you had purchased LR 6 you may need to Sign out of the account that didn't have a subscription then Sign back in with the one you purchased the subscription with. Even if they are the same Adobe ID account using the same exact email address and password.
Do Help / About LR or Help / System Info and tell us what precise version of LR you're running.