You can submit as much as will fit on the media you're sending in. HOWEVER, you should only submit FINAL VERSIONS containing all the different elements of the beat. Which means for example, if you have a beat that only has changes up until the 1st chorus and then repeats, you only need to submit a recording of that beat up until the 1st chorus. To put it another way, if your beat is structured like this:
Different changes:
1-intro part
2-verse part
3-chorus part
and the beat is arranged say 1-2-3-2-3-2-3-fade out, you only need to submit however long is it for 1-2-3..
You can register instrumentals only, lyrics only, or completed songs. If you register an instrumental and record lyrics over it later, you'll need to re-register the work as an entire song.
To answer the original question.. First of all, there is no way to allow people to listen to your beats over the internet but prevent them from downloading local copies. If you're going to post your beats on the internet, I'd recommend tagging them with vocal clips. People are less likely to rip your beats and use them when there's vocals scattered all over it. Also, there's no way to stop someone from recreating your beats. I wouldn't worry about it too much because the chances of someone stealing your music and actually making money off it are really REALLY slim.
Just register all your stuff and hope that if someone ever does steal your shit, they make a million dollars off it so you can file a lawsuit and actually collect payment.