The success of the Android platform is being driven by the sheer number of hardware vendors supporting it at both the high and low ends of the market
It means in this case
sheer - complete and without restriction or qualification; sometimes used informally as intensifier
sheer number means "a great number of". every word has a main meaning but many words have "side-meanings". for example when you look up the meaning of "administration" in the dictionary and try to translate "drug administration" you can translate it in a wrong manner. you should consider the "side-meaning" of "administration". Here, the original meaning of "sheer" is not enough to understand the meaning of the sentence. You should consider the side meaning and then you understand that "sheer number" means "a great number of"...