It should be quite obvious that David D. Porter never let facts get in the way of a good story (or his career, etc.), so anything he says unsupported should probably be "doubted until proven"... as a matter of fact, if there were no contemporary evidence of the fake ironclad(s), I'd have tended to believe that Porter made them up entirely.
From what I can tell, there were three: two (maybe) on the Mississippi and one on the Cape Fear River when Union forces were pushing up past Fort Anderson toward Wilmington. (The latter, called "Old Bogey" in some accounts, was fitted up to look like a PASSAIC-class monitor to draw fire away from the real monitor MONTAUK at Fort Anderson.)
It's not one of my current priorities, but as the sesquicentennial of the event approaches, Porter's stories will likely be told and re-told... it'd be a nice little project to put together the real facts of the incident(s).
IMHO, it's extraordinarily unlikely that Porter had the first dummy put together to cause the Confederates to blow up the INDIANOLA, though that's exactly how he makes it sound. (How prescient he was!) I can't remember where I came across it, but I read a much more believable account that the fake ironclad project was a result of observing the INDIANOLA's passage of the Vicksburg batteries, and was purely intended to make the Confederate gunners waste ammunition (and with luck, maybe burst a gun or two). The INDIANOLA's loss to the South was strictly an unforeseen side benefit.
There's not much that I've seen about the second "dummy" on the Mississippi, and it's a possibility that there was only one... the non-Porter accounts seem to conflict on this point, and I've wondered if it wasn't an artifact of people remembering the dummy at Fort Anderson.
Adding to the confusion is the tendency of contemporary writers to refer to a Union ironclad as a "monitor" and a small casemate as a "turret" (e.g. William D. Porter's "turret" on the CHOCTAW), so the varying descriptions of the "Black Terror" and prospective second Mississippi River dummy can be difficult to reconcile.