As dot says, this is a very difficult question to answer.
Firstly, probably not everything he wrote has survived, so "lost" plays (or other works) cannot be counted.
Secondly, as dot indicates, there are no definitive versions of his plays. In those days once a play was written it was sold to the theatre and THEY owned it.
But the plays (including Shakespeares) were changed and adapted as the were performed and there was often multiple versions of the same play (some lines not even written by Shakespeare but created by the actors).
As dot says, some scripts were written by scribes as the play was taking place and they often got lines wrong, or missed lines out.
Also it is thought that some of his later plays were written in conjunction with other poeple (or maybe not with Shakespeare at all) so how do you count them.
After Shakespeare died his supporters tried to collect definitive versions of his plays and to print them in what was called the "First Folio".
But even these plays had to be pulled together from various surviving scripts and they had to make editorial judgements.
And because the First Folio books were published by different publishing houses, even those plays differ as often the person putting the book together left out words or parts of sentences to make them fit the page.
So all in all an almost impossible question to answer.
http://en.wikipedia.org/wiki/First_Folio