How should i understand the given sentence?

There is an unspecified amount of clothes in the basket. They are dirty. That is all anyone can understand from that sentence. (The plural noun 'clothes' is uncountable. We don't say 'number of clothes').

I suppose that if the "someone" cared at all about the clothes or the basket...the person would ask: Are you intending to tell me that there is an unspecified number of clothes in the basket which are dirty?...or...Are you intending to tell me that there is an unspecified number of clothes in the basket which are dirty, but there are also clean clothes in there?

For someone for whom this information is significant...the original statement, as you suspected, could be interpreted either way.

Personally, I would probably take it the first way, but I would then move on to the question, "And I am supposed to care about that why?"

I'd phrase it this way, if you're asking how best to phrase:

1. There's a collection of clothes in the basket; however, some of which is clean and some is dirty.
2. There is an unspecified collection of clothes in the basket; however, some of the laundry is clean and some is not.

I was wrong in what I wrote above. From this sentence...

There are some dirty clothes in the basket.

... we can understand (only) the following:

There is more than one item of dirty clothing ("some dirty clothes") in the basket.

We do not know if there are any clean clothes in the basket. We do not know if there are any kittens, sticks of dynamite, or lumps of coal in the basket. That information is not present or implied in the sentence.

This sounds like one of those comprehension tests which are used in schools and in job recruitment.

That's the answer I wanted to hear Thanks!

Note: A single unspecified item of clothing is a garment.

If you say: I believe most folks will assume all the clothes are dirty.
If a mixture, you might say:In which case, my wife will wash them all.