When I cancel the creation of a sketch/feature/whatever and restart it the naming system keeps increasing automatically (which makes sense), but I want it to overwrite the non-existing features.
Is it possible to do?
Manually renaming it fix the counting, but still is a boring practice.
I agree it can be tiresome, there’s a couple of habits you can foster to help but I think you are best not worrying about it too much and/or renaming to meaningful names.
Instead of cancelling, try undo.
If it’s a small part you can reload the document quickly.
A macro is probably best solution
You can tidy up your tree with this macro: https://www.codestack.net/solidworks-api/document/features-manager/rename-features-sequentially/ and after it should continue with the next available number as the index for new feature
Unfortunately Undo also increases the counting number 
Usually I ignore it or start renaming from the beginning when I know it will be a complex part; but most of times it is a little complex and just bothers me to stop to fix it. lol
Thank you, worked fine here. 
May I ask any suggestion of site/reading to start learning Solidworks Macros? I just know basic programming and am postponing this. 
My best suggestion is to ignore those numbers. I do it just fine. If you can’t or don’t want to do that, re-name the features, sketches, etc to something that actually describes their function or purpose instead of just using numbers.
Just rename Sketch4 to Sketch3. From then on, then next sketch will be Sketch4.
Try it out.
Usually I name it, but lots of parts is pretty useless to name everything and would be nice to have the names just in order .-.
That’s how I have been dealing with it. lol
thanks 