This feature is part of the Visforms Subscription and not available in the free Visforms version..
We suggest reading the documentation from the beginning, starting with the Introduction. We will not repeat explanations that help you to better understand the whole context.
Enable plugin in plugin manager
Please make sure, that you have installed your Visforms Subcription. After that, you have to enable the plugin in the plugin manager before you can use it.
Go to the Administration Panel of your Joomla! installation and click the top menu item "Extensions" and then "Plugin Manager". Search for the list item with the name "Plugin Visforms - Maxsubmissions". You can use the search filter above the list. Make sure the plugin "Status" is set to enabled.
Configure Limitation Options
These basic settings covered please read the article about the plugin configuration.