Lol it means a high percentage of players. You do not know the percentage but its substantial. So as long as you have been playing for a while and have expanded normally your cities are likely to be included. It is safer to expand towards the centre and it is unlikely that any rim location is included as the sweep is from the centre outwards.
When the era starts the game looks at all cities that have been included and (counting only farm islands) it adds up how many cities are included (that is 100% of cities) and how many each alliance has and calculates what percentage that is.. There is a published running total percentage for each alliance. If the target is 40% then that is what you have to achieve to activate Last Stand.