How does BGP prioritize lesser vs more specific prefixes? And can it even be done?

Hi experts!

We have 2 ISPs. We're announcing our /20 subnet to both ISPs.
I'm wondering if we could announce a subset of this /20.
Does ebgp allow this? can we make as many announcements as we want? (Im thinking if everyone did this, it would fill up the global bgp route list, and may therefore not be allowed?)
Should the subnet be registered at RIPE etc? Is there a limit to how small network announcement we can make?

Lets say it cant be done. How does BGP prioritize this then.
When using static routes, the most specific route is always chosen. Does the same rule apply to ebgp?
It doesn't seem like a normal thing to do, since google is not helping me.

The use case is. Some of our customers prefer 1 ISP over another, so we want to make different announcements based on that.

Please advice. :)