With Proton, any of the apps can be installed on multiple machines. There is nothing difficult about it, just do the install and use it. I rarely use Proton on multiple devices simultaneously, other than logging in on both my computer and phone at the same time, but it's certainly possible. If you use the free VPN, you're limited to one device at a time. You can install on as many devices as you like, but can only use the VPN on one at a time using the same account. There is nothing that I know of to prevent registering for multiple accounts. That's somewhat inconvenient, but you get what you pay for, or less. With a paid plan, you can use up to 10 devices simultaneously.
Proton VPN info
I started using Proton for the secure email service, but I signed up for a paid subscription because of that and all the other features - cloud storage, calendar, password manager (the best I've tried), VPN, et al. It's worth it to me. Free email is not free. Google doesn't charge you money directly, but they get it in the end (or you do). I pay for a package, but it's possible to just pay for a VPN, even though that makes little sense to me because the price difference is so small.
