"Better to buy a few now, and change it every month"
 

Let's say we have a discussion regarding the use of bath towels. The manufacturer recommends using a bath towel for one month and then discarding it. So I want to say " It makes sense to buy three or four now, and change them out every month." I can say it like this:
"Mejor comprar tres o cuatro de una, y cambiarla cada mez."

I'd like to hear other ways to express the same idea.

"de una" is very informal, pretty slangish; I'd use "juntas".

Using a towel during one month and then throwing it away? The worst bachelor apartment ever!