If you'd done a search (or possibly even looked in the stick FAQ posts here) you'd have seen that this topic has been repeatedly addressed.
Despite that, the answer is: it depends on what you need out of your cabinet. Technically for MAME, P1 & 2 don't need more than 6 buttons. Many people put 7, using 2 buttons to send the same input so that they can have a 4 horizontal button layout for NEO Geo games over the top of a 3 button row which will give the 6 buttons needed for Street Fighter games.
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P3 & 4 don't need more than 4 buttons.
Unless you're gonna play 4-player SNES & other later emulators. If so, P3 & 4 may need more buttons.