“All” is one of those words that can go either way. So look at the context!
Here’s an example of getting it wrong:
A handful of buildings and mining relics are all that remains today of the once-thriving mining town of Russell Gulch, Colorado.
The writer got the sentence partly right. The sentence has a compound subject (“handful” and “relics”) and he used a plural main verb (“are” instead of “is”), but he used a singular verb in the noun clause (“all that remains”). Since “all” refers to more than one thing, it’s a plural, so the correct noun clause is “all that remain.”
Things like mass nouns (i.e. uncountable) are singular. You might say “all of the wheat is ground up,” for example.
So be careful with all that you write.
By the way, here’s a picture of some of the town, but not all of it.