I would like to know how many maximum emails can I send from 1 account

Is it 100,
do I need some kind of SMTP server provider to send 500 emails per account? Thank you.

There is no limit in eM Client, but your email provider will limit that. You will need to ask them what they support.

Google Workspace hosted accounts are limited to 500 recipients per message for example.

I guess it’s 50, I tried sending 500+ emails and only 50 actually got sent. Thank you.

There are some mailing list sites that offer inexpensive/free alternatives.
https://www.phplist.com/ is one of those.