So in this context "güzel elbise" would imply specifically singular? As in, "we have a single beautiful dress that we wear in August"?
What's the rule here? Is the plural required whenever you're referring to specific items as opposed to meaning in general? Would you need the plural with a demonstrative too? Like, "Ağustosta o elbiseler giyeriz" for "We wear those dresses in August"?
If you want to talk about a singular noun with an adjective, you need to use "bir." "güzel bir elbise"
If you want to talk about specific things, you just need to use the accusative. It could be singular (just use 'elbiseyi") or plural (elbiseleri).
If you use a demonstrative, you have to use the accusative. "O elbiseleri"