Fisher's formula:

Q.P = M . V

From which: P = M . V / Q, the price level depends on V and on Q.   V depends heavily on the hoarding habits of people, and Q depends on the economic activity.  If M is a hard number, it will never be stable.  Especially if a large part of M is being hoarded, and V is very sensitive to the small amount of non-hoarded coins.

If there's a feedback mechanism from P to M, which is what central banks do, then this can stabilize P.  If no such mechanism is known, P will be very dependent on V and Q.