With the current interface, yeah, naturally. But if you were to redesign the interface, I imagine it would be scalable. That said, redesigning the interface is a lot of work so I'm not gonna complain if it doesn't happen. I just think limiting number of custom configs to a small amount, especially if it's global and not per-chassis, kinda takes all the fun out of a mechlab to begin with. I mean, 20 configs? I could fill that up with just lights.
What would be fine with me, though, is if you could add a couple of your own configs to each chassis, but also overwrite any other configs present in the game (except for Primes) with your own.
Of course this is all a long way off and just speculation, since as far as I'm aware mechlab is nowhere close to ready (if it has even been started yet).