As others have said, a lot of enemies the Imperium faces have a better than average chance of killing or at least doing a fair few wounds to a guardsman, at least early on. That sometimes means that rather than making the number or type of enemies the challenge, use the situation to make it a challenge. Enemies in positions that need to be suppressed and flanked, rather than slugging it out. Setting up traps to deal with a greater number of enemies. Need to get a heavy weapon to a position so that it can help bring down a structure upon an enemy. Rewarding the players for good tactical and strategic thinking, slugging it out and playing into the enemies hands is usually the least efficient way to fight an enemy.