If you're talking about currently, Mutoh has six belts and three titles.
He and Taiyo Kea are the IWGP Tag Champs (IWGP is what New Japan calls their titles--stands for International Wrestling Grand Prix). That's one.
He and Kea are also the All Japan tag champs. Back in '88 the PWF and NWA International Tag Titles were merged to form the All Japan tag belts, so the champs get two belts, like how Chris Jericho has both belts while being the Unified World Champ. That's three.
Lastly, Mutoh's the All Japan Triple Crown holder, which is, you guessed it, another three belts (The PWF, NWA International, and NWA United National Titles, merged in '89). That's six.
Since Mutoh recently jumped to All Japan for good he'll likely vacate the tag titles, bringing him down to "only" five belts.
If you're confused (and God Bless you if you aren't), www.wrestling-titles.com should shed some light.
Hey, remember when Ultimo Dragon had like 10 belts at once a few years back?
I have always liked the "carnival" tournament where all the champions of the same class from the different promotions fought each other in a succession of matches, each winning the other's belts. the last man winning then holds several titles (usually a cruiserweight/light heavyweight title.) Think how awesome it would be to have Tajiri wrestle X-pac, and the Mexican and Japanese Junior Heavyweight Champs and hold 5 titles.
"My parents said I could be anything, so I became an ASSHOLE!"