Additional detectors improve our ability to detect somewhat (assuming they're of similar sensitivity -- otherwise they can actually hurt our overall network sensitivity!). But the _real_ advantage is in source localization to guide multi-messenger (optical/gamma/neutrino) followup, which is where many of the most important discoveries will come from. It's like triangulation.

Given that an observatory costs on the order of ~$1B to build and operate for a few decades, we probably won't see more than five current (second generation) instruments (2x LIGO + Virgo + KAGRA + LIGO India).

There are also two proposed but not yet funded 3rd-generation ground-based instruments ("Cosmic Explorer" and the "Einstein Telescope"), one planned space-based instrument ("LISA"), and early efforts at proposing a future moon-based detector (the Gravitational-Wave Lunar Observatory for Cosmology, the Lunar Gravitational-Wave Antenna, and the Lunar Seismic and Gravitational Antenna).

To get to tens or hundreds of detectors, someone will have to invent a fundamentally different technology that can be produced at dramatically lower cost. Maybe next century...