I would say three, as sanger sequencing is capable of producing on average fragments of 600bp good reads, later the peaks peter out quickly due to low concentration of PCR product. Sanger can produce longer fragments in case if the PCR products are in high concentration. Minimum concentration to obtain a good read for sequencing is 15 femtamoles of DNA. If you have low concentration then the sequencing result will be low. Also make sure you have good overlap between the 3 fragments too.
tray with the primers described in: Int J Syst Evol Microbiol. 2002; 52:1263-6. Probably using 4 Forward and 4 Reverse primers you will be able to cover the whole 16S rDNA gene.