Only three X15 airframes were built, number 1 and 2 did the early flights and number 3 blew up prior to its first flight, it was repaired, and all three flew until Number 2 broke its back on landing. That was also repaired.
They all continued to fly until Number 2 suffered the excessive heating damage after which it was repaired but never flew again.
Number 3 was destroyed when it broke up in the air due to high G loads following a spin, Major Adams killed.
That left number one to make the last flights ending with flight 199 on 24-10-68.

X15-1 56-6670 Air and Space Museum Washington.
X15-2 56-6671 Dmgd/repaired/dmgd/repaired. Wright Patterson Museum.
X15-3 56-6672 Blew up/repaired/destroyed.

A slender delta version was proposed but not built.