at any given moment i probably only have one of each tool on me just for pocket space. if i'm doing a big rehaul of an area though, i probably have spare shovels and axes just to keep me going
I sometimes have an extra tool lying around but it's mostly random. The one tool I always have an extra of (and the one I use the least? wtf, self) are axes.
i stockpile all the tools! some variants for decoration purposes, others to actually use. i like getting the other variants when i visit different islands but as long as i have more than 2 in storage than everything's all good, i worry about not having enough tools when i'm decorating and collecting resources
Lately I have been keeping 1 regular shovel for moving trees/flowers and digging stuff up, and 1 flimsy shovel that is only for hitting rocks (including digging the empty holes for rock hitting purposes). As long as you only dig empty spots and always hit the rock when there are materials in it, the shovel won't break. Hitting rocks does decrease durability though, so it will eventually be at 0 durability, but will only break if you accidentally hit the rock when it's empty or dig something up. In theory it could be at 0 durability for eternity!!