They don't have to designate the SSK class in sequential numericals like 1,2,3,4,5,6,7,8,9. Thus they can always take a numerical that has not been used. Or they could use designation like type 039D/E/F.....Z.(example Song and Yuan class)So what are they going to use after 39? 310?
Thus far in PLAN we have
type 03 for Wiskey class (retired),
type 031 modified Golf class for testbed of submarine launched missiles (retired),
type 032 testbed for SLBM (in current service), often mistakenly identified as type 041 previously.
type 033 for Romeo class (retired),
type 035 for Ming class (mostly retired, others serve in training roles)
type 039 for Song and Yuan class (in current service).
Thus there remains slot for type 034, 036, 037 and 038. What would they do after exhausting all 03X designation is up to speculation in future, may be after 40 to 50 years from now.