Is toast singular, plural, or both?

Bring the toast here! Singular.
The toasts were enlighting. Plural

http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861720608

Whether there is a singular and plural or both, depends upon what kind of toast. If it is bread ( toast) then "toast" can be one or more pieces. If you are talking about giving a "toast" praising someone, then one speech would be a toast and if there are more than one speech, there would be "toasts". If "toast" is being used as a verb, all the verb rules would apply.