Is there a User limit?
Sites created with any of the plans do not have any user limit. However, these sites are limited by CPU hours per day. This means if there are a lot of users using the site, the site may slow down if the CPU usage limit is reached. For a detailed explanation read this blog post on Pricing.
Internal Server Error in my site
More often than not, this error represents some application related issue. If your site is on custom bench, then you can investigate the same with ssh access. It is a possibility that your custom app is throwing an error. You can view the same in web.error.log. Refer our docs for the same.
Can I change email used for receiving notifications?
Yes, you can do the same from https://frappecloud.com/dashboard/settings