I dont think and never seen China ever officially saying they have 300 nukes.
China consistently declined to comment on how many nukes that they got or to refute any numbers being bandied about by
Western knowledgable experts. Perhaps China decided China too stupid in comparision to those folks and declined to join in on their comments.
Perhaps something I wrote here
China Ballistic Missiles and Nuclear Arms Thread might shed some light on the numbers.
Then also consider that while China got 200 ++ nukes as so claimed, there is also a strong possibility China got 2000 to 3000 U235/Pu cores AND 2000-3000 assemblies.
Without the cores inserted, no nukes, or just 200++ nukes. But in time of crisis, China can insert those 2 to 3000 cores into the assemblies in a matter of days. That the warheads delivered be sending more than just dim sum or General Tso sweet and sour chicken or cleaned pressed laundry.