These are you antenna options:
MA-ANT-20, Meraki Dual-Band Omni Antennas
MA-ANT-21, Meraki 5 GHz Sector Antenna, 13 dBi
MA-ANT-23, Meraki 2.4 GHz Sector Antenna, 11dBi
MA-ANT-27, Meraki Dual-Band Sector Antenna
MA-ANT-25, Meraki Dual-Band Patch Antenna
I would personally only use the dual band antennas. A sector or patch antenna tends to be better when you put in on a wall at one and of the building and point it down towards the other end of the building - often down isles of racks.
Omni's are good when you just want to put them in the middle of everything.
I think you should expect to use multiple access points. Hard to say how many without knowing the layout.