I’ve interpreted the situation as whatever format and page count was originally published. For example, Marvels 0 and Epilogue are very short, Wizard 0 type books, whereas Marvels 1 to 4 are larger prestige format length books; 6 in total.
A 52 or 100 page Bronze Age DC counts as one item, and I’d place the long Invasion books that were mentioned in this category as well.
Another complication occurs with Masterwork and Archive collections. In most cases, the original issues with covers are fully reproduced and counting them individually is easy, as with the Werewolf by Night Collection that I read recently. I also treat oversized annuals as one item.
Having OCD, an interesting point to analyse.