I don't know
and when you are about to kill up the first wave and think they can't be trained a second time
your enemy already reached imperial while your economy is beaten and he calls mercenaries once again
plus there are church mercenaries that are called without shipments or population restriction
unlock the hidden church techs without a card
there is one tech that let you trade in some advantage for another, usually you gain a little, and also let you hire
Spain
2000 wood for 10 hackapells /
1000 wood for 13 halberdier with guard upgrade Britain
2000 coins for 26 skirmishers, guard upgrade /
1000 food for 8 highlanders
France (same research, different levels)
500 food for 4 veteran grenadiers, but available in colonial /
1000 food for 8 grenadiers, upgrade also remainings to guard (namely middle guard) /
2000 food for 15 grenadiers, HP & attack +10%
Germany
2000 coins for 12 dopplesoldners, HP +10%
3000 coins and all mercenaries are free now
Netherlands
2000 coins for 30 musketeers, guard upgrade /
1000 coins for 5 stradiots
Ottoman
1500 foods for 3 great bombards /
1000 woods for 15 janissaries
Portugal
2000 woods for 11 black riders /
2000 woods for 22 veteran crossbow PLUS HP & attack +30% !
Russia
2000 coins for 17 hussars with guard upgrades /
1000 wood for 9 dragoons with guard upgrades
just leave 60 pop slots, call a few merc armies and these church ones you can wipe 2-3 players.
enjoy