Guten Tag RobertD50,
Thank you for your question!
Whether the demonstrative pronoun diese is translated as "this (one)" or "these" depends on if it is in its singular or plural form as well as the context.
It is declined as follows:
As you can see, diese appears a lot here. When you are looking at the context, Nik is saying "Ich mag diese (accusative, feminine, singular) hier." - "I like this one here." which implies they are talking about just one blouse.
Paul then goes on in saying "Wir haben diese (accusative, feminine, singular) leider nur noch in Größe 36." - "Unfortunately, we only have this one left, in size 36.“
Lesson 7.5 ‘Talking About This and That’ covers this topic in more detail. It might be helpful to you to take a look at it too: https://members.rocketlanguages.com/members/german/lessons/4724/7-5-talking-about-this-and-that
I hope this was helpful!