Depend on which platform and your art level + plot tbh.
Some people thing 10 subs in a month is a lot they could ask for, some won't be satisfied even if they have 100+. Also the platform/website you are on also determine the average sub count too. Like Tapastic vs LINE Webtoon, where 50+ for a prologue is A LOT for Tapastic, it's not unusual at all for 50+ subs on Webtoon for a prologue, mainly because Webtoon has an average of x3 amount of visitor daily compared to Tapastic.
Another factor is your art skill + popularity before you start making your comic. Take Fisheye Placebo by Yuumei for example, she was already popular on DA and when she made her webcomic, it was phenomenal and naturally, an explosion in sub count. But then you have someone who, maybe have the same art skill, but have no prior fanbase, of course their sub count won't be /that/ high and their comics could often be overlooked.