How many simultaneous sounds can a game play with a Creative Soundblaster Rx?

It does in fact mean 64 simultaneous sounds. More specifically, it means that software can feed the soundcard 64 different sound buffers (probably each one with its own volume and effect information) and it will handle mixing them together into a single¹ stream of sound to send to the speakers.

This doesn't necessarily mean that the soundcard hardware can handle that many buffers. The AWE64, for example, claimed support for 64, but the hardware only supported 32; if you tried to play more sounds than that the driver would mix the extra ones in software.

It's also not a hard limit on the number of simultaneous sounds you can play. Modern systems have so much processing power that mixing extra buffers in software is not particularly onerous. Modern game engines support software mixing out of the box, which also lets them support features the soundcard itself might not like 3d positioning and material-based attenuation. The soundcard may support 64 distinct buffers but any given program might only need one, doing all the mixing in software.

It's true that there were (and still are!) cards available with more, but with the ubiquity of software mixing even 64 is probably overkill; once reaching the point of "enough", soundcards have generally focused on adding more features like reverb effects or 3d positioning rather than merely adding more channels.

¹ or more likely two or more, for stereo or surround

As a postscript: "channel" is a very annoying term, because it can mean:

• a stream of sound sent to a single speaker, like the "front left" channel in a 5.1 setup
• all the sound sent over a single connector, like the "headphones" channel that contains both left and right audio
• a sound sent to the sound card in software, that should be combined with other simultaneously-playing sounds and played back to the user

Generally, I call the first "channels", the second "lines", and the third "voices" or "buffers", but this is by no means universal.