If all goes well, then there will only be one N400 interview which includes the civics/English test.
Right now processing times are all over the place. People are getting interviews 2 months after submission while other cases are waiting 2+ years for interviews at the same field office. The online time estimates are all over the place. Rumor is USCIS is processing some new cases to bring down their average processing times (on paper).
You can re-schedule the interview, but the new date will be totally at the discretion of USCIS, I've seen interviews rescheduled for one week after the original date, all the way to 12+ months after the original date if things fall through the cracks.
Good luck.