The basic Latin alphabet that is used to write Ecclesiastical Latin is as follows;
A B C D E F G H I J L M N O P Q R S T U V X
This is a total of 22 letters. These are all the letters that are needed or used to write native Latin words.
Latin also has 4 additional extra letters that are used in loan words from other languages. These are;
K Y Z W
If you see a word with one of these letters it means that the word is not a native Latin word but rather has been borrowed from either Greek or German or some other language. It also means that even though we are using the word in a Latin sentence, the word still retains its non-Latin identity. This is similar in English when we use words like Fiance from French which retains it non-English identity.
Just as many words from French eventually became "anglicized" and lost their non-English identity, so the same happened to many borrowed words in Latin. We evidence of this in spelling changes. Words from Greek that were formally spelled with a K to identify them as borrowed words eventually lost their Greek identity and became Latinized. People just forgot that these words were borrowed and learned them as part of their native language and as such they took on a Latin identity. When a word such as Kalens was Latinized it changed to Calens, which you may recognize as the origin of the word Calendar.
K and Y represent sounds that are equal to sounds that normal Latin letters make. Z and W represent sounds that do not normally exist within the Latin language and thus can not be converted to a Latin letter. They may on occasion be written by aproximation using DS for Z and UU, or UOU for W.
People who study Classical Latin teach that the letter V makes the same sound as W. Whether or not this is true is not at issue here. What is at issue is that in Ecclesiastical Latin, the letter V never makes the same sound as W.