There are many questions on this site (EL&U) about countable and uncountable nouns, including one about uncountable nouns that can sometimes be used as if they were countable nouns. This question is about its opposite.
There are nouns such as excess for which the plural form excesses is also in use, albeit with a significantly narrowed meaning in this particular case. That excess is a countable noun is further supported by the phrase "there is an excess of ..." sounding more natural than "there is excess of ...".
Now, although excess is a countable noun, it isn't actually countable beyond having one of them - you can have an excess of wheat, but you can't have two excesses of wheat, for example. Contrast this with a truly uncountable noun such as water, where the construct two waters could be read as an elided form of two bodies of water or two glasses of water.
Other related words such as abundance and, less universally, shortage also have this property, as does extent, to an extent.
Is there a more compact term than the phrase itself for seemingly countable nouns that are normally used as if they are uncountable nouns?
Sample sentence: "[Semi-countable] nouns are restricted to the singular."
Update The construct "a/an [xyz] of ..." is often used with collective nouns, e.g. a disguising of tailors. A strange property of collective nouns is that the chosen word (like disguising in my example, or better still, like the amble in an amble of walkers") need only to be suggestive of some property of the group described, and doesn't even need to be nouns at all when not used as a collective noun. I wouldn't normally consider the phrase "an abundance of wheat" to be a collective noun, but perhaps that is at least a step towards an appropriate label.