Change contact setting flood limit
There isn't an interface to change this, but you can do it by importing a config file. You need to get a hold of contact.settings.yml file. Default looks something like this:

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


You can see the flood limit and interval which you can change and then import the file using the config management system.