By what you've posted it looks like all instances would have the same ID.
You'll need to have different IDs and make each call separately (or in a loop).
Alternatively, you could give them a fake class and 'each' them with jQuery. If your class was called
map-canvas-placeholder
you'd do something like this:
$(".map-canvas-placeholder").each(function(){
var currentCanvas = $(this);
});
Whatever function you stick the block above in, that's what you'd call from the code behind.
A Google search on multiple google maps would also yield results.
Cheers.