How is the zakat calculated for the remaining six apartments—whose final purpose has not been determined—if they remain in their current state and multiple years pass, or if all or some of them are sold at any time?

Zakat is not obligatory on apartments that you intend to live in, or on those you intend to rent out or sell but are hesitant about. Zakat becomes obligatory on the rent of apartments if it reaches the [nisab] threshold and a full year has passed over it, and on their sale price if it reaches the nisab and a full year has passed over it. The amount of zakat is a quarter of a tenth (2.5%).