The cpu_count column shows the number of cpu's available
for an instance when the instance started. If number of cpu
changes while the instance is running then the column is not
updated on the fly. Is the latest cpu_count value available
from some other V$ table?
AFAIK cpu_count is not modifiable with alter system or alter session, it can only be modified by restarting instance with new value in v$parameter.
Check the isses_modifiable, issys_modifiable columns in v$parameter. These tell you whether or not the value can be modified with an alter session/alter system command.