How to calculate number of submitted webforms?

Since you wrote you're an absolute beginner, you might want to consider using contributed modules to do so. Be aware however that reporting about webform submissions can be a bit challenging, especially if you try to do so with the Views module. Below are a few alternatives to consider.

Webform add-on modules

There are many Webform related modules which extend Webform, including integrations with third-party software.

As a sample, if you'd want to create charts from the submitted webform results, there seem to be even 2 alternatives to pick from (with extremely similar module names ...):

• Webform Charts, to display Webform results in a graphical manner using a charting library.
• Webform Chart, which integrates with the "Analysis" tab of a Webform (created with the Webform module) to provide charts for each individual component's analytical data.

Another possible module to consider, is the Views Aggregator Plus module. Here is an excerpt from its project page:

Works great with Webform 7.x-4.x (aggregate your webform submission results tables).

Module Forena

Display Webform Submission Data using Views is a bit challenging. As an alternative to installing Webform MySQL Views (which depends on Data and Schema), consider the Forena module. Here is a quote about it:

... built of the idea of using SQL to get data out of a database and use XHTML and CSS to format it into web reports.

So in this case you'd use Forena to report about the Webform data stored in the Webform tables, which are contained in your Drupal database.

Disclosure: I'm a (co-)maintainer of the Forena module,
I hope this does not violate the site's policy on self-promotion.