Think of the radio transmitter like a can of paint that explodes inside a hollow ball (sphere)
It fires a signal (the paint) out over an ever increasing area (proportional to the distance from the transmitting antenna).
Imagine you're using a mythical isotropic antenna and you'll see that all the available paint is evenly spread over the inside of that hollow sphere -- and the larger the sphere, the thinner the paint (signal strength) will be.
Now in order to receive the transmission, each receiving antenna is effectively taking the paint from a small area of that sphere's inner surface.
So long as none of those areas overlap, no antenna will interfere with another, nor will it cause a greater load on the transmitter.
In effect, the number of receivers is limited by the area on the inside of that sphere -- because that's all the paint (signal) there is.
So the potential number of receivers is infinite (because your sphere can be made larger as required) but you soon reach a point where the amount of paint (signal) is so small on any given area that it can't be seen for the background noise.
This is also why we use high-gain antennas to extend our range -- they allow us to effectively collect more paint (signal) over a wider area than a lower-gain antenna.
Sorry if this is a bit patronizing but I'm used to writing simplistic explanations of complex topics as part of my job so sometimes I can't help myself
