"They are buying eight dresses altogether."
I think it is implied in the word "are". The English can be interpreted two ways: that they are currently at the cash register buying, or that they intend to buy. The 要 would be in regards to the latter interpretation.
要 has many meanings (the "pop up" lists given by Duo are just the basic/ common meanings) 要 can also mean "going to" ("are buying" here).
Because the object of this sentence is 'dresses' I sort-of figured 'they' might be female. Sadly, 她们 is not accepted.
In Taiwan, "裙子" is "skirt," and "dress" would be "洋装" (traditional: 洋裝) yángzhuāng literally "foreign attire."
I think that word means both "dress" and "skirt" in Malaysia. I might be wrong though
Once again, there are half a dozen ways to arrange this sentence in Chinese, but Duo only recognizes one.