How many apps would it take to reach 90% of the people in your country?
For example, I live in Thailand and the (Japan-based) messaging app LINE could probably reach almost everyone with a smartphone.
Seems like in most Western countries it would probably take 3 or 4 apps? What are they where you live?
Probably one... the stock messaging app? In the US, most people send and receive SMS text messages like it's still 1999.
One, in Denmark, we have a system called MitID (basically SSO login for public services (and private services can use it too). Even though there are other options (hardware totp dongle and such), most people have the app installed - I would guess easily over 90% (over the age of 13).
One. WhatsApp is pretty big in Turkey