We have been running the HL2DM server, Source of Horrors, at 14 slots today and it is entirely playable :D

dm_lockdown dm_darkmatter_b2 dm_np_refinery_b7 dm_outpost_final dm_frenzy dm_lambda_bunker dm_lostvillage dm_morbid dm_swamplight_b3 dm_substorage termination

Please let us know if any of these maps cause lag at 13 - 14 players and we will remove it.
We'd like to identify a solid set of maps for 14 -16 players.