@Mrowell 

It depends on how you use the extenders.  

Using as Traditional Extender:

Multiple extenders can be used but they cannot be daisy-chained (connected to each other).  They each need to be linked to the Host router or AP and then places in opposite directions of that host Router/AP.  For example, if your Router is in the center of your home one extender could be placed east of the router and another west of it. This also means that once you setup a Range Extender you need to unplug it to set up the 2nd one other wise you run the risk of it linking to the first RE.

Range extenders in traditional mode also are half-duplex devices.  They can only communicate in 1 direction at a time.  This means to the router or to the client.  Because of this speed from a Range Extender can be up to 1/2 of what the router provides.  In most cases, a range extender cannot provide better speed, only better range.  This is also why you do not want to daisy chain, as the 2nd extender will further reduce what the first extender provides by up to 1/2

OneMesh/EasyMesh:

For most of our newer extenders, there is an additional option of operating in a Mesh mode.  This is achieved in one of two ways not.  1.  Through our TP-Link branded OneMesh.  This allows you to use a OneMesh-enabled router with compatible extenders to make a Mesh network, where you have multiple nodes broadcasting a single network SSID and providing seamless wireless all over your home. 2.  EasyMesh does the same thing but does hold you down to just one brand.  EasyMesh is a universal standard so any EasyMesh router or extender can be used, even if its not TP-Link.  Either way, you can also daisy chain, though we don't recommend going more than 1 jump between extenders.  Speed on the remote nodes (extenders) will be less than the main node (router) but not as bad as in traditional mode. This option only works if your router support EasyMesh or OneMesh though.  

Whole Home WiFi:

If you don't have an EasyMesh or OneMesh router or your extenders don't support the feature another option is going with a  Whole Home WiFi system.  This is a full mesh system like our Deco products.  It is usually sold in multi-unit packs of 2 or 3 units but can have more added as needed.  Like an EasyMesh/OneMesh setup, you would have one unit act as the main node (router) and the others would be Satellites (remote) nodes.  They can connect to each other via wireless or Ethernet and would also cover the home in a seamless network.   This is Ideal for customers as its easily expanded and very easy to control through a single universal app.