Particle photon accessing ONE variable - is there a limit to number of users listening?

[Moors7]

Unless you change the URL, you're still using the variables.

@marklee

My particle publishes data to a variable

You don't "publish to a variable", but rather you publish data (which can be a variable) or you request data that's been assigned to a variable.

Publish = push
Variables = pull

Have a look at this tutorial: Tutorial: Getting Started with Spark.publish() - #27 by bko