Moved both get_num_instances_for_*let to a single Extension.get_max_instances.
Moved both get_num_instances_for_*let to a single Extension.get_max_instances.
Making single instance the default for all extensions
Merge pull request #1905 from Lusito/maxinstances

Moved both get_num_instances_for_*let to a single Extension.get_max_instances