Vmix has built in the ability to stream to 3 destinations at once. Due to facebook's api user agreement you have to use "custom destination" instead of the regular facebook destination if you stream to facebook and another destination at the same time.
If you need more than 3 then you'll need to use some sort of re-stream service

Keep in mind that you will need to have enough internet bandwidth for each individual destination. (i.e. if you are streaming 3mb per destination you will need at least 9mb upload speed)