Will adding Ethernet shield reduce the IO count of Arduino?

Question

I have searched this question but could not find an answer on this site.

There was a related question - Latest Ethernet shield - used pins, how to bypass restrictions on arduino forums, but, I was unable to understand what the answer meant.

The background of my project:

My project requires

| S.No. | IO Type | IO Qty |
|-------+---------+--------|
| 1     | AI      |     11 |
| 2     | DI      |      9 |
| 3     | DO      |     11 |  

I intend to connect this arduino board via ethernet shield to a network. I want to receive data over the network for display and storage purposes on a PC.

Questions:

1. What Arduino board should I select?
2. Will ESP8266 be compatiable with the suggested model?
3. Are there any pitfalls that should be considered when making the selection for this design.

P.S: I have raised the same question on the arduino forum. If I get an answer- I will share my answer here as well

Answer 1

You should use a Mega if you really need so many gpio pins.

Arduino Ethernet and WiFi shields and their clones use 3 GPIO pins with SPI function with connection on ICSP header. Next they use the Slave Select signaling pin 10 and the SD card Slave Select signaling pin 4. The shields are compatible with Mega. So you should still have enough pins.

esp8266 can be connected to SPI. then it uses the same pins like the Ethernet shield except of the SD card slave select pin.

if you connect esp8266 to Serial port, then all the pins are free. Mega has 4 Serials.

esp8266 is cheaper but you need to connect it with level shifters and have a good power supply. there is no good esp8266 shield for now. The Mega + WiFi board from Robotdyn has the esp8266 on board with Serial connection.