@RMDG
Hey,
For your questions, hope my answers can give you some insights.
1. can i use 1 cpe220 as a.p. (receiver from office) for 2 cpe220 client (from the field machine area)..?
Normally yes. But you have to make sure 2 CPE220 clients are within the coverage angle of the AP.
2. or do i really need to use wbs + sector antenna as receiver if will have 2 or 4 cpe as client?
Nope. WBS + sector antenna is basically designed for ISP with large network deployment. For most home users and small business, CPE can do a good job.
3. will the cpe220 as a.p. (receiver from office) able to received signal if we put a cpe from the back side?
CPE uses built-in directional antennas. You need to face to the front of it. The signal of the back will be much weaker or even no signal if the distance is too far.
This document of common applications may help you understand PtMP:
https://www.tp-link.com/us/configuration-guides/common-applications-for-pharos-products/?configurationId=22033
Hope it helps!