How can I draw a donut chart based on the number of specific fields on a form?
Yes it's possible to create such charts (which are called Donut Charts) similar to the sample in the Interactive Google Charts gallery (move your mouse pointer over any of the slices in that sample to see the special effects it comes with also). Such Donut chart is actually a Pie chart with a hole in the center, which can be created with the pieHole option.

Below are some more details to get you going ...

Option 1 (without using WebForm):

Use the Comparison of charting modules for potential alternatives to consider.

Some of the modules mentioned there:


Charts, which can be used to Create charts using Views ... as in these Charts previews and/or its demo site. The D7 version of it comes with a choice between 2 supported charting libraries to select from.
Chart, which can be used to Create charts using Views ... as in these Chart previews. It requires the (deprecated) Google Image Charts, and is expected to be upgraded in the near future to use another charting engine. Charting engines being considered are either of these JS libraries:


The JIT (Javascript InfoVis Toolkit).
The d3.js library.



Option 2 (Using WebForm):

There are actually 2 alternatives to pick from:


Webform Charts (used in about 400 sites), to display Webform results in a graphical manner using a charting library.
Webform Chart (used in about 220 sites), which integrates with the "Analysis" tab of a Webform (created with the Webform module) to provide charts for each individual component's analytical data.


The Drupal 7 version of both modules, use the Charts module to actually create the charts (module dependency).

Remarks

The license that comes with the charting library (engine) that is used in the selected Charting module, should be considered also. Some of the typical items to consider are:


Possible privacy concerns.
Flash based libraries (as they were also used in the D6 version of Charts) should be avoided if charts are to be displayed on mobile devices.
Google libraries require a working internet connection (not always available in local DEV environments ...).
Not all libraries are free of charge (some come with commercial licenses).


Disclosure: I'm a (co-) maintainer of Charts and Chart, and the author of that Comparison.