Answer
About 50 servers. In review, anything from 33.3 to 75 counts as right.
The worked estimate
Usable capacity per server: 2,000 × 0.6 = 1,200 requests/s. 60,000 ÷ 1,200 = 50 servers. Add a few more so that losing a machine (or a whole zone) still leaves enough.
Why
Running servers near 100% makes queues, and so latency, grow sharply; a target utilization of 50–70% leaves room for spikes and failures.
Numbers: Numbers to know.