So okay I'm not sure what maps are already on the server, so I couldn't do the math, but here's a list of the maps used in lobbies via tf2center.com I hope I got them all, there's not like a comprehensive list anywhere so...
cp_badlands
cp_coldfront
cp_croissant_rc11
cp_dustbowl
cp_freight
cp_foundry_final1
cp_glassworks_rc5
cp_gorge
cp_granary_pro_b10
cp_gravelpit
cp_gullywash_final1
cp_logjam_rc2
cp_metalworks_rc7
cp_mountainlab
cp_process_final
cp_snakewater_final1
cp_steel
cp_sunshine_rc7
cp_warmfromt
cp_warmfrost_rc1
ctf_2fort
ctf_ballin
ctf_ballin_comptf
ctf_ballin_sky
ctf_bball2
ctf_bball_comptf
ctf_bball_stadium_a3
ctf_doublecross
ctf_turbine_pro_rc4
koth_airfield_b7
koth_arctic_b3
koth_asheville_rc1
koth_brazil_rc1
koth_coalplant_b7
koth_harvest_final
koth_lakeside_final
koth_product_rc8
koth_ramjam_b9
koth_resevoir_b4
koth_stallone_b2
koth_ultiduo_r_b7
pl_abandoned_upward_finalfix
pl_badwater
pl_badwater_pro_v8
pl_badwater_rainy
pl_badwater_rainy_nowater
pl_badwater_snowy
pl_barnblitz_pro6
pl_borneo
pl_goldrush
plr_hightower
pl_swiftwater_ugc
pl_thundermountain
pl_upward