The only stuff that we have on WiFi in my house, are the following: An Android phone, two iPhones, a Nook with the n2a Android software micro-SD in it, our thermostat that uses Wireless-B for the radio. Everything else is wired. The total devices on our network is at 15, most we have had on it was 21 devices, with no problems. We have used our Roku wireless with no problems. Then again Netflix is not that fast.
Personally, when pulling Ethernet, take the number of drops and multiple by four for future proofing.