How do I set collectables?

Hello community, I know the title has no sense Xd I'll try to explain.

I've created a platform. I want to insert some collectable object.

For example among the game I want to insert 30 "power stones".

When you reach the final boss, If you have all the 30 "power Stones" there is an alternative ending.

I have problem with the "power stones" count.... I mean if I collect 5 stones in the 1st level and, for example, I die at the middle of the 2nd level, the game is set to "restart the layout" but I want the "power stones" count to be saved in order to restart the level with the exact number of stones I've gathered in prevoius levels...

I don't know if its is enough clear :D

And another little problem.... in the pause menu I've set a slide to control the master volume. If I set the slider to half bar, when I die the layout restarts with the volume at the max level.

How can I store the level set?

I hope someone can help me.

Thank you for your patience and understanding :)

You would use a global variable to track the number collected, but also you would need 'Persist' behaviour to keep the ones you collected as destroyed when you restart the layout.