For multiple nerves treated with mulitple injections you bill 64421 with quantity one. It is not per injection
Below is CPT Assistant article and lay description from Encoder
Ignenix Encoder
The physician anesthetizes the intercostal nerve to block chest wall pain. In 64420, a single injection is performed. In 64421, multiple nerves are injected to provide pain relief to a larger area (regional block).
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CPT Assistant November 2010
Surgery: Nervous System
Question:If a single injection is performed for a thoracic intercostal nerve block, the effect of which is to block multiple intercostal nerves, it is appropriate to report CPT code 64421,Injection, anesthetic agent; intercostal nerves, multiple, regional block,or is the provider limited to re-
porting code 64420,Injection, anesthetic agent; intercostal nerve, single?
Answer:From a CPT coding perspective, “single injection” means “single nerve block.” Therefore, code 64420 should be reported. If only one needle is placed into the skin but manipulated up and down to inject into more than one nerve, it may be possible to report multiple blocks (code 64421). However, from a clinical perspective, blocking multiple intercostal nerves with a single injection is hard to do in most settings.