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Vitamins only add 10 EVs in their respective stats up until you've reached 100 total EVs in that stat. So if you have 0 speed EVs and you haven't used up all 510 EVs yet, you can use up to ten Carbos before you stop getting the effects. If you have 60 speed EVs already and then decide to use Carbos, you can only use up to four Carbos.
Hope that helped.