In almost all cases, phrases like "those jeans" or "your jeans" will refer to a single pair of jeans. If the speaker wants to refer to more than one pair, they'll add extra words like "all" or "always" to add this context. Examples:
1a. Are those jeans on sale? (one pair)
1b. Are all of the jeans on sale? (all the pairs in the store)
2a. I like your jeans! (the one pair you're wearing now)
2b. I always like your jeans! (every pair I have ever seen you wearing)
Sometimes that context might be provided with gestures instead of words. If I asked a store clerk, "Are those jeans on sale?" while making a sweeping gesture with my hand across an entire shelf of jeans, then that would mean that I'm referring to all of them. If I'm pointing with my hand not moving, then I'm probably pointing at one pair.