Translation:They are buying eight dresses altogether.
A lot of times I've heard 要 used in the sense of "I'm intending to do this and there's no question that I'm going to" (basically I will...). In Wiktionary under the pronunciation yào the 2nd definition is "will; going to" so 要 just has multiple meanings that we're used to having separate words for in English (but we do have uses like "It is the king's will that the traitor should die" where the meanings of want and will are rather mixed)
My understanding is that 条 means a strip of something, and is used as the counter or measure word for long, thin things, including dresses, pants (trousers), also rivers, etc. So, when it's used with pants, it is often translated as a pair of pants, but that's a bit misleading, as pants are singular in some languages and plural (i.e. a pair) in others. Here it is not translated as a pair. If one insisted on translating it, the result would be something like "they want to buy eight articles of dresses," which isn't good English. (Am I making sense?)
Both "want to buy" and "will buy" are correct here. You need context to properly translate this one. The English "altogether" (entirely, completely, etc.) is confused with "all together." A better way to be clear (lacking context) would have been to use "想" (Xiǎng) meaning "to think" but in this case would be "to think to do (something)." A double compound needed for character number balance is 想要Xiǎngyào (to want, to desire to).