How many number skillstone in this game? It's very hard (or maybe impossible) to unlock all characters skills because skillstone are very rare items.

It will be better add a shop to buy skillstone.

You get an abundance if you do all content/explore it all. Quest bosses start dropping them. Dorgania is really rich with them. I had enough to max out 4 people and get the party wide passives like Trey Def, HP from servo, Agility Brad, Luck from dragon,etc.. I did not grind the Spire. You could. Sara has skills to guarantee drops or steal them over and over. Either rapid kill them end game with hit alls or sit there and feed her TP potions to rob over and over.

Just beware some dungeons are missable/one time thing. Like dragon mom quest which is good to do early as it can give 50% exp bonus to someone. Save regular and don't progress to end point. There will be a end game fort with a missable one too.  Left path is a skillstone, right one is a HP 200 item. Ignore the bad advice of not opening them.