I've installled EM Cloud Control 12c on a linux 6.2 box.
4 database targets are being monitored. One of then is an Oracle 22.214.171.124 database (EE) 64 bits with 4 cores. However, the "Performance Home" page shows 1 core only. I'm a little worry, because I've seen on this page that the CPU use is 100% (1 cpu) and I think Oracle is not using the other 3 CPUs.
You may want to cross-check the number of CPUs seen in the Database homepage with the number of CPUs shown in the (database's) host homepage. On linux, the number of CPUs is derived from /proc/cpuinfo, so check the file too if necessary.