"She only has a shirt."
Translation:Αυτή έχει μόνο ένα πουκάμισο.
The Greek sentence of this exercise means that there is someone who either has one shirt and nothing else or does not have >2 shirts.
An example using "απλώς", similar to your suggestion:
-Ρε φίλε, τι φοράς; (=Dude, what are you wearing?)
-Χαλάρωσε, απλώς φοράω (ένα) πουκάμισο. (=Relax, I'm just/only wearing a shirt.)