The success of SpaceX Falcon Heavy on its first launch was not just luck. Although this will be confirmed in 2019 and 2020 based upon what happens with about five planned Falcon Heavy launches.
Before the Falcon Heavy flight Musk predicted a 50 percent to 70 percent chance of success because of concerns over the difficulty predicting how the vehicle would respond to extreme aerodynamic stresses and vibrations from the clustered engines.
SpaceX has a 96.88% launch success rate (62 out of 64) with the Falcon 9. This launch success is with five major design changes for the Falcon 9 rocket.
There has been 84% success on landing first stages (31 out of 37). All landings in 2017 and 2018 have been successful other than the loss of the center stage for the Falcon Heavy.
There has been 100% success on re-flights (17 out of 17) of boosters.
SpaceX is learning more about accurately simulating the performance of rockets prior to launch. They are also understanding how to change rockets and still have successful launches.
I would put the over and under for the number of launch failures during Starship Super Heavy testing at two. It is 50-50 or better odds for two or fewer launches to fail.