last time i looked it up it was up to 100K active troops including reserves after that your not classed as a Defense Force . .If you have armed Police force they are also counted as they have weapons training and could be  called up for active duty in line of defense or offense. but there is no  actual class of armed neutral nation.eg Ireland is a DF and we don't engage in front line warfare unless our Troops are fired at in there AO on Peace keeping duty on the other hand Australia is also classed as a DF . Where they are a more offense DF which spent time in Afghan and Iraq with US and UK Troops. It really just depends on the constitution or the individual Country  and what there Military is allowed to do from the get go.Ireland in our  case we cant attack another state unless it goes though the Dail then  there need to be a Vote by the People then the president sign off on the  change to our constitution. then the military can mobilize.Also being  Neutral dose not restrict you from joining alliances for defense.  example Ireland is a member of the Nordic Battle group. which is a EU Defense Alliance that is suppose to be able to Mobilize in 72hr anywhere in the EU. constitution loop hole