Can not place rollback segment online

Dear all DBAs,

i'm using Oracle 8.0.5 on Linux 6.4 kernel 2.2.14. when i try to place the rollback segment online, it show error as below:

SQL> ALTER ROLLBACK SEGMENT "RB28" ONLINE; ALTER ROLLBACK SEGMENT "RB28" ONLINE
*
ERROR at line 1:
ORA-01599: failed to acquire rollback segment (28), cache space is full
(currently has (29) entries)

If use oerr ora 1599, then
the solution it showed is to "take another rollback segment offline or increase the parameter max_rollback_segments"

so that means the current rollback segment number is exceeded the max_rollback_segment? where's this max_rollback_segment?

Thanks in advance.

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