Depends on the games you plan to play -- For example if you want to play Robotron, Smash TV and similar you would want 2 joysticks - for fighting games like Street Fighter etc. probably 6 buttons rather than 4. THen there are the specialized games that use Spinners or Trackballs. So best to decide what games you are really wanting and then design the CP layout to fit those games.
For Example I like to play Robotron and Smash TV so built my CP with 2 joysticks and also do use some of the fighting games so also included 6 buttons and added 6 other buttons ( 1&2 Player Start and Coin, Pause, Exit) Also using a ZD encoder so wired it so that the 2nd joystick and player buttons 3-6 share the same input since it is limited to 16 inputs ( 4 for joystick 1, 4 for joystick 2 shared with 4 buttons, 2 unshared buttons, and 6 Admin buttons = 16 inputs) and no games use both the 3-6 buttons and 2 joysticks so those were the easiest to wire as shared inputs !)
I purchased a set of joysticks and 14 buttons from Ebay for $33 + $10 express shipping which along with the $11 ZD encoder was all I needed for the setup including 1 button I used for the on\off switch on the back and left 1 extra button not used so about $50 total investment.
EDIT : Be sure to order the correct type of ZD encoder for the buttons\joystick you are getting -- I notice the one you posted is designed for a Japanese Joystick that uses 5 wires and also the smaller Japanese quick disconnects for the buttons rather than the Happ type Joysticks and buttons -- so be sure to get the type that fits the buttons you are getting !!
Here is a pic of the layout I used for my Bartop :
