We can not give you an exact number, development of the product is carried out continually and we release new version as soon as we have concluded a particular set of changes that constitute a useful release. 

But as a general guide we issued 3 releases last year, this does vary year on year as function of the complexity of each release. There could be more or less in the future, the demand for new functions from customers and our own plans for improvements fuels these changes. 

For a historical perspective have a look at the release history page for a clearer picture of events.