1.3.6.1.2.1.25.3.3.1.2.1 = CPU0
1.3.6.1.2.1.25.3.3.1.2.2 = CPU1
1.3.6.1.2.1.25.3.3.1.2.3 = CPU2
1.3.6.1.2.1.25.3.3.1.2.4 = CPU3
...and so on.
Here is some working code from my setup:
### "^" Prepended text
### "$" Apended text
### "_" Default text
Title[^]: CPU Usage (%) on System:
MaxBytes[_]: 100
Options[_]: gauge, absolute, nopercent
Unscaled[_]: dwmy
YSize[_]: 100
YLegend[_]: CPU Usage (%)
ShortLegend[_]: Percent
Legend1[_]: CPU Usage (%)
Legend2[_]: CPU Usage (%)
LegendI[_]: CPUUsage: 
LegendO[_]: CPUUsage: 
Directory[_]: novcpu
####### End Defaults
Target[CHA_CPU1]: 1.3.6.1.2.1.25.3.3.1.2.1&1.3.6.1.2.1.25.3.3.1.2.1:public@172.17.202.16
Title[CHA_CPU1]: CHA
PageTop[CHA_CPU1]: <H1>CPU #1 Usage (%)
on System: CHA</H1>
Target[SNOOPY_CPU1]: 1.3.6.1.2.1.25.3.3.1.2.1&1.3.6.1.2.1.25.3.3.1.2.2:public@172.17.202.17
Title[SNOOPY_CPU1]: SNOOPY
PageTop[SNOOPY_CPU1]: <H1>CPU Usage (%)
on System: SNOOPY</H1>
Legend1[SNOOPY_CPU1]: CPU0 Usage (%)
Legend2[SNOOPY_CPU1]: CPU1 Usage (%)
LegendI[SNOOPY_CPU1]: CPU0Usage: 
LegendO[SNOOPY_CPU1]: CPU1Usage: 
CHA is a single-CPU system, where SNOOPY is a dual-CPU system. I don't have anything more than 2-CPU systems right now, so I don't have experience with getting MRTG to accurately display a 4-CPU system.