How many tweets per second does Twitter need to ingest?

The scenario

Your team is designing the write pipeline for a Twitter-scale platform. The service has 100 million active users posting 500 million tweets per day.

Traffic is not evenly distributed (spikes around events, timezone peaks), but for capacity planning you use the average sustained rate.

What is the average tweet ingest rate in tweets per second?

Adapted from: donnemartin/system-design-primer — Design Twitter Timeline — CC BY 4.0