We need an estimate of the messaging traffic volume (on average and during spikes) that the push infrastructure will need to handle. This will depend the number of users expected to opt in and the kinds (and frequency) of events we need to support.
- Android active devices (as of 09 Mar 2020): 7.7M
- iOS montly active users (as of 18 Feb 2020): 3.8M
- Estimated push notifications opt-in rate (iOS): 43.9% 
- Estimated push notifications opt-in rate (Android): 91.1% 
- Pageviews (Feb 2020): 20B 
- Edits (Feb 2020): 44M 
- User edits (non-bot) (Feb 2020): 21M 
- Estimated push notifications opt-in rate: 5-20%?
- This estimate will depend on how many users we decide to prompt, which will depend on in what situation(s) we do so.