Wat exact antenna you get depends on your application.
While connecting 6 individual antennas is possible, you typically would get the antenna to match your radios.
Since the AP is 4x4 5GHz with 2x2 2.4GHz that's what your antennas should support. This could be something like a AP-ANT-40 for example.
Keep in mind antennas need to be mounted aswell and hence might require additional mounting options.
If you have little experience with the AP and (external) antennas, I would definitely check with your local Aruba representative. Have them double check what you come up with or have them suggest what you need even.
Since it is omnidirectional you need, you might be able to save yourself some headache with some AP with built-in antennas like AP-365 or AP-375. They realy do simplify installation and design typically.
Some additional reading: https://support.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-a00044653en_us.