Who was the first player to score a double-century in ODI? How many balls did it take?

The little master, India's Sachin Tendulkar. His 200 took just 147 deliveries, with 25 boundaries and three sixes. South Africa were the opposition in a day/night match played on 24 February, 2010 at Gawalior in India.

The ODI (One Day International) cricket format is played over two innings of 50 six-ball overs (300 deliveries). Each bowler is limited to delivering 10 overs. The side with the most runs wins.

From their 50 overs, the Indian batsmen compiled a daunting 401 runs for the loss of just three wickets. Tendulkar was left not out on 200. In reply, the South Africans were bowled out for 248 in 42.5 overs. India won by 153 runs.

https://www.espncricinfo.com/records/most-runs-in-an-innings-216972

https://www.espncricinfo.com/series/south-africa-tour-of-india-2009-10-428634/india-vs-south-africa-2nd-odi-441828/full-scorecard