i made a hair particle system and then made real instances of it. Then when i try to join them using ctrl j the whole thing changes and the same objects gets multiplied many times. pls help I'm using blender 3.3
All instances share the same mesh data after converting. When you perform the join operation this mesh data changes - it becomes consisting of all meshes you have joined. But as soon as all objects initially used in the particle system continue sharing the same mesh data - which now is the joined mesh - each of them starts to look like multiplied mesh.
To avoid it, before joining meshes, first go to the
Object Data Properties tab (green triangle icon in the bottom right panel) and press a digit near the object data name. This digit determines how many objects in the project share the same mesh data-block. By pressing it you make a copy of the mesh data block of the active object and assign this newly created mesh to this object. This way the active object starts using its individual mesh data-block and you can join other meshes to it without affecting other objects which used the same mesh data-block.
You don't have to make individual data blocks for each object you are joining, only for the active one.