re writing articles

How many 500 word articles can a normal writer rewrite in 8 hours ?

Depends on fluency, I guess 1 article per 30 minutes

normal writer can write 5x500 words articles per hour.

there is a software called dragon***** try it

do you rewrite one sentence at a time, or do you read the whole article and then rewrite in your own words?

Make sure your re-writing has a focused purpose, that will make things go faster. If you aim at creating enough content for 2 micro-niche sites a week, for example, that might come to 10-12 articles/posts per site, and if the work is spread out over 6 days, means writing 4 originals a day. Writing one set of re-writes for each set of originals, or another 4 articles a day. At a half hour per original or re-write, that 4 hours of work a day. If that's realistic for your normal rate of output or capability, go for it---or if not, cut it in half or so, to create a more sustainable rate of labor.