The number of 5th gen fighters that China needs to consider would be dependable on its economic growth. F-35s and T-50s have to be purchased from their own manufacturing countries and the buying countries may have limited capabilties in producing the parts needed while China could produce most of the parts and weapons systems for its indigenious fighters programmes except perhaps, for the engines itself. The complexity and superiority of the fighters will need to be taken into consideration too. You do not need the same number of air planes to match your opponent's air force. An advance fighter with superior weapons may match two advance fighters with lesser advance weapons. I am not saying which fighters are superior, just making a hypothesis. The model would also changed as the procurement blocks goes by.
Also, even if a country may order say 40 units of F35s or 40 units of T-50, it may take some time to manufacture, assemble, testing, flight tests...etc and then to delivery. In short, the deal may perhaps stretch over a period of "several" years, say 4 years. So in 1 years, only 10 aircrafts may be produced; you must also factored in some other issues such as social and political factors which may hinder the production of these aircrafts. As compared to China who could produce indigeniously in a year, hence over a period of similar years, they might be able to produce more aircrafts depending on their industrial capabilties, which again the engines might be a crucial factor.
In my humble opinion, China needs to maintain the its 'backbone' fighters such as in the likes of J-11Bs, J-10Bs and JH-7s. These 3rd gen would still be doing most of the defending works for the PLAAF and PLANAF. The more advance fighters (speculated) in the likes of J-20 and rumoured H-8 bombers would be doing the attacking orders. Hence, in order to match/counter the air superiority of other countries who have F-35s, China need to introduce a fleet of 5th gen fighters with the $$$ which they got. As of now, we do not know the cost/unit of J-20, J-31 and other new unmanned armed/unarmed vehicles so it is not very wise to speculate the number of units they might purchase.
From Gen. Lin Biao's most famous quote, "Attack the direction where they feel its safe!"
Against a country with superior air power, you would not do anything stupid as to provoke a war against them. However, if the other party do, that is how the H-6K with superior CJ-10 cruise missiles would do. These missile could be fired away from the range of most AA weapons and do the job by disabling as much of these advance fighters as possible.
“When strong, avoid them. If of high morale, depress them. Seem humble to fill them with conceit. If at ease, exhaust them. If united, separate them. Attack their weaknesses. Emerge to their surprise.”
― Sun Tzu