Before its destruction, was the Crystalline Entity one of a kind? Or is there information leading one to believe that various Entities were in existence at some point?
Within the TV canon, there was only one Crystalline Entity (e.g. the one seen in the episodes Datalore and Silicon Avatar). There are several lines in the second episode which confirm that it's the same creature as in the first:
DATA: Trace elements confirm that it was the same entity which destroyed the colony on Omicron Theta.
The crew also make it pretty clear that it's unlike anything the Federation has encountered before.
DATA :... confirmed by Lore's report to be an unknown kind of creature, capable of stripping all life from an entire world...
GEORDI : It's like a snowflake crystal, sir. But much more complex... I haven't the slightest idea what it is, sir.
Moving further down the canon scale, we see numerous crystalline entities in the Trek novel Orion's Hounds.
Troi turned to Oderi, who was back on her feet and consulting the status-display band she wore about her wrist. “What is it? What’s going on.”
“The station is under attack by branchers.”
“A very, very deadly form of starbeast. Huge living crystals. They fire beams that disintegrate living matter and absorb its bio-energy. They—”
“Oh, my God. Can you show me an image?”
Oderi nodded and tweaked her status band to project a small hologram of the scene outside the station. Deanna gasped at the sight of the familiar, ramified blue forms closing in on their position. They were the same kind of Crystalline Entity that had destroyed Data’s homeworld decades ago.
And there were three of them.
There's also a much bigger specimen seen in the Star Trek Online episode "The Crystalline Catastrophe". It takes roughly ten sovereign-class vessels to kill it.