According to a report published in the latest issue of the journal ACS Chemical Neuroscience by experts Jean-Louis Reymond and Mahendra Awale, it would appear that the number of “small molecules” available for synthesizing new drugs is one novemdecillion (1 followed by 60 zeros).
Put into common terms, that's 1 million billion billion billion billion billion billion small molecules. This class of molecules contain substances that can potential penetrate cellular walls, and produce biological effects within. In other words, they are chemicals that could be combined to create new drugs.
The number of potential combinations exceeds even the estimated number of stars in the entire Universe, and there are very many stars out there. The Milky Way alone has about 100 billion, EurekAlert
At almost 67 million substances, we've only managed to synthesize less than one tenth of 1 percent of all potential combinations. This means that we have an extremely long road ahead, but one that could yield tangible results in the form of new therapies against some of the most devastating conditions out there today.