If current version of ruleset if 99-99-99, and still need to create new version for the development.
Do we have a Pega best practice policy about this situation?
what should we for the new ruleset version when current version is 99-99-99?
I do not think in real life such a scenario has ever reached. Usually in case when a 99 version is reached, skimming is recommended to move to the next version but in your case it is not possible. The best case scenario is to raise a ticket with Pega GCS and follow their recommended steps. You can get more information on Rulesets and Skimming in the following links:
https://academy.pega.com/topic/rulesets/v4#p-e60b557d-3a8e-48c1-bd6c-5e9de08b4854
https://docs-previous.pega.com/rules-management/87/rule-skimming-higher-ruleset-versions?