Hi, is it necessary create a extra user for each device send mqtt message ? or can i use the same user/pw for every device ?
pb1966
One user for everything works, however it can help to have one ‘server’ and one ‘client’ account.
My personal preference is to separate different types of devices - e.g. one account for tasmota devices, as explained more here:
Clients are separated by the topics they post to MQTT tele/tasmota-1234, but share other topics such as to upgrade firmware of a group of similar clients cmnd/tasmotas/upgrade.
If this helps, 
this post!