An HSA rollover may take place only once a year. Technically a rollover is when you receive a distribution of your HSA funds personally and you must put them into another HSA within 60 days. You can only do that once a year. Transfers of HSA funds directly from one HSA trustee to another HSA trustee is known as a trustee-to-trustee transfer. You can do that as many times as you want.
Source: IRC Sec. 223(f)(5)