Replace-Selected-Objects-With-MultiInstance.py

import c4d
from c4d import gui

#This script required Cinema 4D R20 and upper

def main():

    objs = doc.GetActiveObjects(0)

    if not objs:
        gui.MessageDialog('Please select some objects!')
        return

      mcs = []

    doc.StartUndo()

    for obj in objs:

        if obj == objs[0]:
            continue

        mcs.append(obj.GetMg())
        doc.AddUndo(c4d.UNDOTYPE_DELETE,obj)
        obj.Remove()

    ins = c4d.BaseObject(c4d.Oinstance)
    ins[c4d.INSTANCEOBJECT_RENDERINSTANCE_MODE] = 2
    ins[c4d.INSTANCEOBJECT_LINK] = objs[0]
    ins.SetInstanceMatrices(mcs)

    doc.InsertObject(ins)
    doc.AddUndo(c4d.UNDOTYPE_NEW,ins)

    doc.EndUndo()
    c4d.EventAdd()

# Execute main()
if __name__=='__main__':
    main()