I've seen people have issues with only 30 pages, others with many more - if your comic isn't split in chapters, just go where the stopping point feels the most appropriate and see which page quantity(including cover and back) won't break the bank. Always keep in mind that you might want to keep your comic's original files to account for bleed if you ever have plans to print.
I have the 1cm black border around my pages for this purpose, at first to give as a freebie on an Artists' Alley when I didn't have more than 3 pages and character profiles, but I'm still doing the pages the exact same way because no art gets lost in the bleed.