No. If, when you are adding a provider in Therabill, you mark them as being an assistant, then an NPI is not required. If the assistant does have an NPI, you can enter it, but it does not go out on the claims (the supervising provider goes out as the rendering provider on the claims).
An assistant can get an NPI from the NPI registry at https://nppes.cms.hhs.gov/NPPES/Welcome.do by registering under the taxonomy code 174400000X for "specialist". But again, the NPI is not required for billing insurance in Therabill.