What is the frames limit of the mirrorless cameras sensor? When should it be changed (for example on the Sony a6000 series or on the Sony a7)?
The sensor on a digital camera is a solid-state device, and while theoretically degradation is possible, in practice, they last basically forever. Don't worry about it.
On the other hand, the mechanical shutter on these cameras does have a finite life, probably normally in the range of 100,000 actuations (although it's not typical for camera makers to give a number for lower-end cameras). But, normally, this isn't something you'd proactively change out — you'd keep using it until the shutter fails, at which point you'd either have it repaired (if you've used a newer or more high-end camera heavily) or just replace the camera.