Collisions

Two blocks of mass $m$ and $M$ $(m<M)$ are initially still.

We push the block $M$ in the direction of the block $m$.
The block $m$ will hit the wall and bounces back toward $M$.
After a certain number of collisions of $m$ between $M$ and the wall, $M$ will move to the right indefinitely.

Given that $m=1,M=123456789$, how many collisions are there?

It is assumed that the collisions are perfect: no friction, no energy loss.