Adding the billing card to the Heroku account and cons/pros of using dynos

Question

Since I am not very familiar with Heroku I need opinion of a specialists like you. I got two questions to which I cannot find an answers on the web.

1. Heroku tempts with the 450 dynos if I add the billing card. Is that secure to add it?
2. Will they take the money from my account without my permission? If for instance I have used all the dynos, would they take the money from my account to add some dynos?
3. Do dynos last for long? How many users can use my site if I got 1000 dynos? I tried to understand the answers from the another questions here in stack, but I could not understand them.

Please do not use the technical language, try to answer these questions as simply as you can (for me and users who will read it).

Answer 1

This is a very vague question, so I'll do my best to answer it clearly.

1. Adding your credit card to Heroku is perfectly safe. They are a legitimate company.
2. Heroku will automatically charge your credit card at the end of the month if you go OVER their free tier, meaning you use more than 450 dyno hours across your Heroku account, or if you provision paid addon services.
3. Dynos last forever, until you remove them. Heroku allows you to add dynos, remove them, etc., all instantly.

You also asked how many users your site can support if you have 1,000 dynos: this is equivalent to asking how many users your site can support if you have 1000 web servers. This is dependent on many factors: what does your site do? How was your site built? Etc.. In general, most webapps on Heroku only require 1 dyno to run indefinitely.