To how many bluetooth receptors can I send sound from my MacBook?
I have used aggregation to play sound via Bluetooth and wired audio at the same time. I originally used soundflower. But you can use apples AudioMIDI app which does the same thing.

Apple have instruction here.

How to combine multiple audio interfaces by creating an aggregate device

Which basically shows you how to create a single output selection that sends the output to more than one device.

I am unable to test this against multiple Bluetooth devices but think if there is any way to do it then this would be it.