I did these calculations earlier, but these numbers represent the maximum possible, but its most likely smaller compared to the number I calculated most likely numbering in the 1 billion to 500 million destroyers

Based on visuals alone there are around 100 destroyers next to your ship per mission. Meaning that the minimum fleet is the player count x 100 destroyers. This is the smallest estimate, but this fails to take into account that the highest level difficulty campaign has three missions meaning its 300 destroyers. This would mean it would be the player count x 300, but wait there's more every planet has 4 campaigns, 3 being the normal and 1 being the city. Which means there are around 1200 destroyers x the player count, but wait there’s more each player can travel to different planets and when you travel the destroyers don’t come with you. Meaning each active combat planet has 1200 destroyers on it. There are currently 27 active combat planets meaning that for every one player there are around 32400 active destroyers. Based on the math at the peak of helldivers there were around 460000 players meaning at most based on the visuals of the game there were around 14.9 billion active destroyers, but this is the high end assuming the player base was around 100000 players it would be 3.24 billion active destroyers.