Every pattern is different. Check the pattern for the required yardage in the weight they recommend. If you have worsted at 190 yards, and the pattern calls for 1100 yds worsted, you’d need to buy 6 skeins. 6x190=1140 yds. If you switch out the weight, going down to DK or up to bulky the size of the item will change accordingly (assuming that’s something you’re willing to doing such as with a scarf). More difficult to sub when dealing with a garment. The same number of yards in DK won’t give you the same size/drape/look as worsted, if that’s what the pattern calls for.
Does that offer some insight?
ETA: also note the fiber content. You might not want to do a cardigan in 100% cotton as it’ll get heavy and there might be shrinkage issues. But a cotton tank might be just ideal for warm weather. You might not want to opt for 100% wool if you’re planning to toss the item in the washer/dryer on reg basis but it might be perfect for throw/afghan. Make sure the fiber content is consistent with the item’s use.