It is well known that the Poisson distribution is appropriate for calculating event probabilities - from Wiki below:
Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since the last event.
In the case of MH370 debris finds the lambda value, based on eight pieces over 16 months, is calculated to be 0.5 (starting with the right flaperon in July 2015)
So the trivial calculation above yields the following:
P(0) ~ 0.60 // probability of no confirmed or likely pieces being found in any given month
P(1) ~ 0.30 // probability of one confirmed or likely piece being found in any given month
P(>1) ~ 0.10 // probability of two or more confirmed or likely pieces being found in any given month
Of course, the rate of debris finds is highly dependent on the level of search activity. The above numbers assume it will be the same going forward as it has been in the past i.e. not very much.