If you have 2 “Human-Wave Tactics” then both effects are applied, so if 1 Level 2 or lower Normal Monster is destroyed in battle, then you Special Summon 2 monsters.
If your monster is destroyed as a result of battle while on your opponent’s side of the field, you can’t get the effect of “Human-Wave Tactics” for that monster.
If you don’t have enough vacant Monster Card Zones to Special Summon as many monsters as you need to for “Human-Wave Tactics”, Special Summon as many as you can, and the rest are destroyed. This is also true for multiple “Human-Wave Tactics”, so if you have 2 “Human Wave Tactics”, and each wants to Summon 2 monsters, and you only have 3 Zones, Special Summon 3 monsters and destroy 1 monster.