The noun nucleotidylexotransferase can be countable or uncountable.
In more general, commonly used, contexts, the plural form will also be nucleotidylexotransferase.
However, in more specific contexts, the plural form can also be nucleotidylexotransferases e.g. in reference to various types of nucleotidylexotransferases or a collection of nucleotidylexotransferases.