You can score a total of 4 Yahtzees
I am used to the 'official MB Games' version. From memory, you get 100 extra points for every 'extra' Yahtzee that you can legitimately fit into another category, so you could theoretically get a Yahtzee in every category except Low and High straight (a Full House being 3 of a number and 2 of the same number again.)
If you are intending to develop the game, it might be cool to have a multiplayer version (over a network ??), as player v. player tactics do come into play when you are chasing an opponents score... Do you take the 'safe' low straight when you still have a roll to go, or risk a no holds re-roll to try for another Yahtzee...
An alternative challenge would be to try writing some AI code for a computer opponent.
I'm sure the community will be happy to chip in with help and advice on either of these options ;-)