They say you shouldn't charge from 0 to 100 often because it can reduce the health of your battery.
Yes. It will. If you do it every day. But, sometimes it's okay.
What about charging from a low number (20-10) to 100%
It's better than 0-100%. And when device reachs 90% , it trickle charges. So, it won't damage the battery.
is it bad to do short chargings often?
Not at all. In fact, doing small top ups are completely fine.
I try to keep the battery around or above 40% whenever I'm out.
I try to keep as much as I can whenever I am outside.
Only exception I can think of is OnePlus Dash charger. It doesn't heat your phone that much when it's charging.( Even when it's fast charging.) I hope iPhone will also implement something like this in future.
Read this for detailed information.