Do I retain licenses if I cancel monthly plan?

You, at a maximum, accumulate 12 months worth of credits. They will be forfeited when you cancel. Licenced images are kept available for use in your account and may be used as long as you can respect the licensing terms. Downloading may not always stay available as Adobe or the contributor may take down the asset. That happenes sometimes, eg when the asset exhibits quality problems that were not seen when the asset was approved.

So you could use your asset credits for licensing assets you may need and cancel afterwards.