In some parts of these worksheets you have multiple letters that can be used to make word, but pay attention to the pictures that are provided. There is much debate as to how many letter sounds there are. It is commonly accept that there are 40-45 distinct letter sounds. The fact that there is debate over this, when you spell a tough word incorrectly should make you feel better.
An interesting fact is that the letters "Y" and "W" can and many times serve as vowel sounds. But there are 16 consonants that have consistent sounds they are called constant consonants.