That really depends on a few things. First, the placement of the Nest WiFi units makes a big difference. If you can arrange to have the primary Nest WiFi Router unit (the one that connects to your internet service via Ethernet) placed as close to the center of the home as possible (i.e., the middle of the second floor), you may find it covers the whole home by itself. If you need more coverage, then you can place a Nest WiFi Point unit in the middle of the first and third floors to provide coverage to more distant clients. That will sacrifice peak performance in exchange for a larger reliable coverage area.
Beyond these placement concerns, the construction materials used for the walls and floors will have a big impact. If it's a wood frame construction, that will be easier to deal with than concrete floors and/or brick/plaster/concrete walls. If you're dealing with dense materials like these, then you'll need a lot more access points, and you'll need to run Ethernet to each of them. A Google WiFi system can work in that environment, but other companies make solutions that may be better fits as well.
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