Below is from AMA CPT Changes 2012, these codes are broken out between cervical/thoracic (64633, 64634) and lumbar/sacral (64635, 64636).
64633 represents the primary procedure for the first cervical or thoracic level, which would be abstracted from the procedure note based on the facet joint level or the medial branches that innervate that joint. As seen below, whether for example, it states radiofrequency ablation of L3, L4, L5 or L4-L5 and L5-S1, you are counting by facet joint level, not individual medial branches. RT or LT modifier or 50 modifier should be used. The T12-L1 joint would be a thoracic level. The add on codes (64634 or 64636) would represent additional facet joint levels addressed.
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AMA CPT Changes 2012
......the unit of service is a single facet joint in new codes 64633, 64634, 64635, and 64636, rather than a vertebral level. If both facet joints at the same vertebral level are treated, then the parent code (64633 or 64635) should be reported with modifier 50 appended. It is important to note that the number of nerves injected for a single facet joint does not affect code selection. Therefore, the new codes indicate "nerve(s)" in the descriptors.....
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Cervical or Thoracic
64633 Destruction by neurolytic agent, paravertebral facet joint nerve(s) with imaging guidance (fluoroscopy or CT); cervical or thoracic, single facet joint
64634 cervical or thoracic, each additional facet joint (List separately in addition to code for primary procedure
Lumbar/Sacral
64635 lumbar or sacral, single facet joint
64636 lumbar or sacral, each additional facet joint (List separately in addition to code for primary procedure)