@ChiefyChief 

IoT devices won't use so much bandwidth, so it's not a problem for router. ER605 can handle 100 normal devices, ER7206 can handle more;

However if all of these IoT devices require wireless connection, you will need to pay more concern on wireless Access Points. From what I know, most of the IoT devices can only support 2.4Ghz.

And because of the limitation of 2.4Ghz, one AP can only connect limited client devices, for example one EAP can connect 30 2.4Ghz clients refer to this article:

How to choose EAP products

That means you will need to install many APs, maybe one AP for one room.

Note Mesh won't increase the wireless client numbers. For a Mesh network, the client device number depends on the main root AP, no matter how many mesh AP added to the network.