How long is a Welsh word?

Retina, 23 bytes

i`[cprt]h|dd|ff|ng|ll|.

Try it online!

Even moar regex.

05AB1E, 24 23 21 bytes

Code:

u•éÓœ°D¥M™ù>•30B2ô0:g

Explanation:

u                      # Convert the input to uppercase.
 •éÓœ°D¥M™ù>•30B       # Compressed version of CHDDFFNGLLPHRHTH.
                         It convert the text between the •'s from base 214 to
                         base 10 and converts that to base 30.
                2ô     # Split into pieces of 2.
                  0:   # Replace each element that also occurs in the input by 0.
                    g  # Get the length of the processed input.

Uses the CP-1252 encoding. Try it online!

JavaScript (ES6), 44 bytes

x=>x.match(/[cprt]h|dd|ff|ng|ll|./gi).length

The trivial answer may be the shortest.