It is possible to make infinite amount of backpacks out of one backpack.

To do that you need to put a backpack in your hand, then press right click on the mouse to equip it like you do with a piece of armor. that results with one backpack on you, and another backpack in your hand.

the version of the plugin is BackpacksRemastered-2020.0.5, and the server is an Aternos server using Spigot 1.16.5

This is only in creative mode and a feature of Minecraft itself. Try it in survival, and it wont work.