Its a weak inflection. Plural for Accusative ends with -en. I am also learning and making mistakes, but I have to admit its fun :) http://canoo.net/inflection/gr%C3%BCn:A?darj=0lang=enview=splitformInput=gr%FCn
I'd say it's any plastic/glas/china cup/mug that has a handle on it. "Tasse" can be small cup or a large cup/mug. A "Glas" is necessarily made from glas, but may also have a handle. You may also use the word "Becher", it may be used for large mugs with handle and for cups/mugs regardless of their material as long as they don't have a handle... Quite complicated I have to admit :)
It has to do with what determiner (article or similar) you use.
When you have a definite article (der/die/das/den/dem) and some other ones, then the adjective is weak.
https://en.m.wikipedia.org/wiki/German_adjectives#:~:text=The weak inflection is used,the adjective endings are simpler.