[Man-of-the-Wold]

The rout of Faramir from the causeway forts and the subsequent siege definitely involved a number of Nazgul, whether all nine are accounted for, I do not recall, but I think not, and it is quite reasonable to assume that Sauron had a few them in Gorgoroth; remember, Frodo and Sam saw one alight at Cirith Ungol from down below.

At the point of the Battle a few may have been in flight to check up on Saruman, again, or to direct armies issuing from Dol Guldor, or from there/Brown Lands into the Wold, all of which were diversionary actions to pin down potential allies of Gondor, and not serious campaign's comparable to the assault on Minas Tirith. I doubt that any Nazgul was sent as far as Erebor.

Please note, though, the Nazgul are not brave warriors. Rather they are cowards. There strength is in lonely places or in causing the seeds of despair to grow. They are no doubt a model for the Dementors of Harry Potter. The Witch-Kings attack on Theoden and Snowmane was a fittingly underhanded manuveur.

In the face of determined charge by a force of Men, their power is marginal, and their flying beasts are clearly vulnerable to prepared archers.

Also, the entrance to Minas Tirith was the Morgul Lord's big moment, and the others were subservient to him, and he may have sent them to the rear for his own hubris.

A short-time later when he was destroyed against all odds, it must of greatly unnerved his brethren. They might have been temporarily left ineffective without their leader and the evident emergence of someone capable of harming them.

This last point would speak to Gothmog not being a Nazgul, but I tend to assume he is, and that they were not rendered completely out of it, but that it was all that Gothmog (and whatever comrades he had with him) could do keep their forces marshalled and organized from the rear, and that like Gandalf, they no longer had the opportunity to directly join in the frey.