Contact email flood limit is per whole system(application) or user/ip based?
As you can see its number of emails a user  can send in an hour 

https://www.drupal.org/docs/8/core/modules/contact/overview#controlling-repeat-submission-blocking-flood-control

You can specify the text to display on your site-wide contact page and you can optionally limit how many times a user can submit a contact form in an hour.


btw, its defaults that you can override per any contact form . for example :
contact.settings.yml

default_form: feedback
flood:
  limit: 5
  interval: 3600
user_default_enabled: true