Number of links
This is a simplistic view of links because what quality the links have will probably have a greater effect on rank, but assuming the quality and distribution meets an expected pattern, then number is probably counted as a relevant measure.
Link numbers would likely be calculated on a page level, domain level and subdomain level, where page link count may contribute to domain link count, but not to subdomain link count which operates on its own page and domain link count formula.
I believe that link quality, link source, link anchor text, link-from page context, linked-from site context, linked-from site genre, linked-from site location etc will all have greater impact on the consideration of page-rank received through links than this simple view of total number.
In short – it’s not necessarily about the volume of links but rather the quality of those links, but both are important. Here’s Neil Patel’s view on that.