Depends on the crystal spec. Many 32 kHz crystals made for low power oscillators are specified for 9 or 10 pF loads.
If it is a leaded IBP tubular package (like seen in computer motherboards) I would guess 10 pF load. Don't forget to take PCB and IC capacitance into account. For 10 pF net load, try 15-18 pF on each side to ground (3-5 pF across Xtal for IC and PCB strays).
Avoid ground plane below crystal pads, PCB runner, and IC input pins as these can add a couple pF to loading. If you are using a raw CMOS inverter you need a series resistor on output to xtal as too much drive can destroy crystal.