When do we use 'revision' as a countable noun?
When revision means a change, it can be countable, as in make a few revisions to a report.
When it means examining something so it can be changed, then it can be both: a system in need of revision AND a revision of standards.
Finally, when it means learning for an exam, it is always uncountable.
Like most derived nouns, revision can refer to a number of things. In the case of revise (the verb that revision is derived from), at least the following senses can be distinguished:
- Revision can refer to the process of revising: Some revision was necessary in Chapter 5.
- Revision can refer to 1 or more events of revising: There have been six revisions so far.
- Revision can refer to 1 or more products of revising : This is a revision; I want the original.
(2) and (3) can be combined: I have all six revisions in my collection.
Only in sense (1) is revision a mass noun; in the other two senses it's a count noun.
Revision may be used plural e.g revisions; but singular form revision may also denote a set of changes. So, at least it is safe to use plural form if you are talking about several sets of "change(s)". E.g:
The plan underwent constant revisions.
On the other hand, with the meaning "study" or "review", you may use revision singular as in
I have to do some history revision tonight