Crimps have a color code that dictates the combination of conductors.  Two is the maximum allowed in one crimp

These are not allowed at all.

Splitbolts are not allowed on a grounding electrode conductor.

I always try to twist the crimp.  If it moves at all....so slight that you can't see it but you can feel it......then it is loose.  With this one the wire fell out.

Only one wire is allowed in the terminal.

Crimps require a specific tool ..... pliers is not that tool.