The answer to this question lies in the time it takes to process the pages sent in order to convert them into PDF/a. If this operation is too cumbersome, technical time-outs will be reached and generate an error.
A distinction needs to be made between pages from a word processing application (such as Word, Pages, etc.) and scanned pages, which are more akin to document ‘photos‘.
In the first case, and provided your documents do not include images, you can easily have documents of 500 pages or more signed.
In the second case, it depends whether you work in black & white or colour, but you won’t exceed 100 pages in black & white.