I'd be curious to see stats on this. At least in the US, it is very easy to buy a SIM and sign up for a pre-paid plan with zero KYC.
And that's a quite high regulation part of the world, I'd be surprised if South American or African countries were stricter.
https://www.comparitech.com/blog/vpn-privacy/sim-card-regist...
But it looks like the official answer is 36:
> Those without any SIM-card registration requirements are Bosnia and Herzegovina, Canada, Cabo Verde, Comoros, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Iceland, Ireland, Israel, Kiribati, Latvia, Liechtenstein, Lithuania, Maldives, Malta, Marshall Islands, Micronesia, Moldova, Namibia, the Netherlands, New Zealand, Nicaragua, the Philippines, Portugal, Romania, Serbia, Slovenia, Sweden, the United Kingdom, the United States, and Vanuatu.
Also facebook/instagram/whatsapp/telegram/etc are not trying to advertise themselves for the high risk use cases Signal is actively promoted for. I obviously do not recommend anyone use those either, regardless.
Matrix is all I suggest for most people.