Lightroom mobile monthly subscriptions?

A subscription to a Plan from Adobe is a 12month Contract. You can pay Monthly or Annually.

Early cancellation (eg. after 2 months) of a subscription contract incurs a $ penalty (50% of the remaining 12month contract).

https://helpx.adobe.com/creative-cloud/faq/CCPp.html

Lightroom-Mobile Apps are available in both the Plans shown at the base of this link-

$9.99 PHOTOGRAPHY PLAN (USA)

Lightroom Plans

The 'Free' Lightroom-mobile from the Apple Store has 'In App' purchase options for the "Premium" versions that provides CLoud storage and more editing  features- (Prices change by Country)