The noun 'toast' is a mass (uncountable) noun as a word for a food substance. The mass noun 'toast' is a word for a form of bread that has been browned by heat.
Example: Would you like one or two slices of toast?
The noun 'toast' is a countable noun as a word for an occasion when people raise their glasses and drink together to honor someone or something.
Example: I made a toast to the bride and groom which was followed by several more toasts.
NOTE: Some mass nouns for substances do have a plural form reserved for 'types of' or 'kinds of'.
Example: The toasts on the menu are rye, cinnamon, and garlic.