It feels a lot like the Global Hawk program. The first seven airframes were built as an Advanced Concept Technology Demonstration to evaluate the design and demonstrate its capabilities. At the time, there was such a high demand for the Global Hawk's capabilities that demonstrator aircraft were pressed into service in Southwest Asia. Also, the Global Hawk entered initial low-rate production while still in engineering and manufacturing development (EMD).
So, we have nine RQ-180 aircraft built for EMD. First flight was in 2010 and operational test and evaluation began in 2014. The first production airframe joined the fleet in 2017 and by 2020 there were at least four or five others. I would imagine that a few of the EMD airframes may have been retired by now and one reportedly crashed in 2016. More production airframes may have been built since 2020, so the total fleet numbers remain an open question. It would make sense for the RQ-180 to be a significant part of the B-21's "family of systems" that has been mentioned from time to time.