Hola Marcos

Interesante... 

It is very very common to say "un par de pantalones", and when you say it this way it's actually a bit unclear whether you are referring to one or two trousers. I personally would say: tengo que comprarme un par de pantalones if I need "two" trousers, but some people may use it to refer to just one. I imagine the word "par" is used sometimes to refer to one because there are "two legs" and this could be the justification for "par". 

However I think it's safe to say that if people say  "tengo que comprarme UNOS pantalones", there is no doubt here that what you need is just one. 

Saludos

Inma