Simply use at least 30gb to avoid possible future issues.In fact I suggest 50gb.This is all assuming usual software & no large pc game.
So do I need atleast 32Gb free disk space on C, or does it automatically install over existing, with only 10 gb free space?
No, the installer will fail. I had 29GB free and the upgrade failed because of an insufficient space.
Up till 1809, my wife's laptablet with a 32GB emmc drive, could perform the upgrades with only 6-7GB free (not reserved, i have to clean up her traces to be able to get the upgrades done). I didn't upgrade her laptablet to 1903 (yet), because it's off limits for me, to tinker with
On a NTLited install with Reserved Storage disabled used space is 7.20 GB here. On a untouched ISO install it was over 2 GB more space used. If you apply the WIM to HDD instead of using the Setup you could install on 15 GB free space or less. I Installed onto a 10 GB VHD and it boots fine.
Yesterday installed windows 10 1903 here, it needed around 23GB free space. After that you can reclaim it back with disk cleanup. I used the Windows 10 upgrade tool for it, because it is Enterprise version. I had an external usb 3.0 drive connected, and it did use this station to save the iso it downloaded. It then also installed from this drive. After finish it cleaned al traces from the install. So installing from an usb drive is the best and fastest option! Now have OS Build 18362.116 Enterprise installed.
did you used ntlite to disable Reserved Storage ? can you tell my where to disable it ? i clean install 1903, and i lost 7gb on ssd for RS, so i fix that by clean install old version 1703 with iso file on my pc, then i upgrade by using 1903 iso file, now i have 1903 18362.30 without RS
Yes, I integrated a reg tweak to disable it but NTLite has an option now to disable it, I guess the option is not available in the free version.
NTlite probably loads the reghive of the mounted wim and uses the exact same regtweak as you did manually. https://forums.mydigitallife.net/threads/reserved-storage-in-windows-10.79347/