Can i set more undos in zbrush ? I can do 4 undos by default, but i need to set at least 15, if possible. Is it possible and if yes how is it done ?
It is possible through the Preferences>Mem menu. However, you’re asking for disaster if you do that. Every undo requires more and more memory. A value of 15 would force ZBrush to always keep 15 versions of your model dynamically in memory, plus the one you’re working with. Expect crashes, and failures when you try to save the model.
The Undo settings specify the minimum number of undos that ZBrush will keep available. 4 is the recommended value. You can try going a couple above that, but 15 is nuts. 
ZBrush will always try to keep more undos than that available. But when it reaches a point where the number of undos in memory would cause performance or stability to suffer, ZBrush starts eliminating the extra undos. It won’t go below the undo setting no matter what, though.
There is another trick that you could do. This is not the best option but when you are in a real bad need you can use this. ZBrush is always recording so if you go to ZScripts and click End Rec. Save this file.
Then load it back into ZBrush under the ZScripts palette.
Remember to hit play on the bottom.