Answer
About 2,300 requests/s. In review, anything from 1,150 to 4,600 counts as right.
The worked estimate
10,000,000 × 20 = 200,000,000 requests a day. A day has 86,400 seconds, about 100,000, so 200M ÷ 86,400 ≈ 2,300 requests/s on average. Peak is usually 2–3× the average, so plan for 5,000–7,000.
Numbers: Numbers to know.