This would be used when someone has to memorize certain information, but the nature of that information is difficult to grasp and recollect or a large set of numbers. Is there a particular word for these?
The question is somewhat broadly focused on both memory and comprehension. Addressing "difficult to grasp or recollect" as in the first example, a thing that is difficult to remember in that way can be referred to as "elusive".
Referring then to "a large set of numbers", as in the second example, a word for difficult to remember due to size or volume (i.e. a large list or long passage of text) is a "challenge". It can be a "challenge to remember", a "memory challenge", or simply "challenging".
Perhaps saying the data is boring or dull or dry implies that it is hard to remember.
If you describe something such as a book, play, or activity as dry, you mean that it is dull and uninteresting. [disapproval]
...dry, academic phrases.
A lot of the work was very dry and boring in Westminster.
Perhaps esoteric will do the trick? When used broadly, as in bit b of definition 1 on Merriam-Webster, it can mean 'difficult to understand.'
The first definition of Recondite on M-W is 'difficult or impossible for one of ordinary understanding or knowledge to comprehend.'
Or, for something a little less esoteric, how about a simple:
Complicated, which can mean 'difficult to analyze, understand, or explain, definition 2.
I know that your request is inclined more towards 'difficult to remember', but since understanding and memory intermingle, I wager that these words could work in the provided circumstance.
Alternately, I like @Gustavson's 'memory-demanding.' Let me know if my efforts helped some.