Although it is possible to run win 10 on 128gb, we normally suggest more as it gives people space to grow into.
I had a 250gb ssd on last PC and always had 187gb free on it, but I am not like most people. I had almost everything on a hdd instead as its way easier to recover from reinstalls if you already have all your data. I refuse to only have one drive.
I was going to do the same on this PC but I have so much space now on C drive that it didn't seem to make any sense to use hdd for a lot. I still have a 3tb hdd but its mostly empty. 44gb lol.
500gb is plenty enough for windows and anything else really... just leave at least 10% free or the ssd will slow down.