The possibilities of earning Item Tokens will vary according to the kind of Toolbox you are opening. 

If you want to check the exact percentage on the go, you can always tap on your toolbox and then on the info button near its title. :) 

Nevertheless, here you have all the Toolbox options, Zoobster!

Bronze Toolbox
All runs will have 20% of chances to get you Common Item tokens and 80% Character tokens.

Silver Toolbox
All runs will have 80% of chances to get you Common Item tokens and 20% Rare ones. 

Gold Toolbox
All runs will have 80% of chances to get you Rare Item tokens and 20% Epic ones. 

Emerald Toolbox
Each run will grant you 2 item tokens per character. So the first token will always be a Rare Item, and the second one will have 50% of chances to be Rare and 50% to be epic.

You can also get item tokens from the Legendary Crate!

This is the only crate that can get you an item. This only happens if you do not draw a new character.

The number of items you receive will vary depending on the amount of characters you own.

For example: You have 16 Characters. You will receive 16 epic item tokens distributed among 5 characters.
You have 7 Characters. You will receive 7 epic item tokens distributed among 5 characters.

This is it! We hope you open lots of Toolboxes and Crates, Zoobster! See you at the Zoo!