Use LD 21 to see what values are currently stored in CFN0, CFN1, etc:
LD 21
PT1000
REQ: prt
TYPE: rdr
TYPE RDR_DATA
CUST 0
TYPE RDR_DATA
CUST 00
OPT CFO CFRD PVCD CWRD
FNAD FDN
FNAT FDN
FNAL FDN
CFTA YES
CCFWDN
CFN0 4
CFN1 2
CFN2 1
DFN0 4
DFN1 2
DFN2 1
MDID YES
NDID YES
MWFB YES
In this example, CFN0 is set for 4. That means any phone that has RCO set to "0" will ring 4 times. CFN1 is set for 2. Again, any phone with RCO set to "1" will ring 2 times.
In other words, the RCO value is just a POINTER to the RDR table. You only have three choices, 0 or 1 or 2.
Now, if you want a phone to ring 9 times (which is absurd, in itself, since most callers give up after around 6 rings), you have to change (use LD 15) one of the CFN values to 9. Bear in mind that this means ANY phone with that CFN value in RCO will ALSO start getting 9 rings.
Hope this helps.