What's the meaning of 'sheer' in this context?
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
More here.
In this case it basically means "large" or "overwhelming". The usage is somewhat idiomatic; sheer is often used in the place of "complete" or "unadulterated", and, from that correct usage it's sort of slid into an all purpose adjective to indicate scale.