The term memory is both a countable and uncountable/mass noun.
When we speak about one's capacity or ability to retain information we can speak about having
a "memory" for something. The OD provides these examples of usage
I’ve a great memory for faces
She still has a great memory for all the old Irish songs and poems.
We can have one or more memories of the past
(sing) What happened during that week was just a bad memory in the past
(pl) I have no recollection of my past memories, except periodic flashbacks of my previous life.
The noun memory is singular when it means "mind". It is not generally used in the plural.
(sing) Williams searched his memory, trying to remember what he did in this situation eleven years ago.
It is a mass noun when we are remembering or honouring the life of a dead person.
A candlelit vigil took place in Huyton last night, one week after the alleged assault, to honour the dead teenager's memory
and when we speaking about a certain length of time
After one of the most hectic holiday seasons in recent memory, many of us have settled in for equally hectic work schedules.
If we were speaking about our childhood, and we retold delightful stories about our past the listener might say
- What lovely memories you have!
Which could be shortened to
- What memories!
If we were speaking about a single incident that happened over fifty years ago, when we were three years old. The listener would exclaim in admiration.
- What a great memory you have!
- What a memory!
The phrase might refer to the actual recollection or to the speaker's capacity to remember something that happened so long ago.