@ccie_ma the MR18 has two 2x2:2 radios and the 600Mbps is the combined raw data rate. The MR18 is also capable of using 40MHz channels and the document is for high density deployments where the use of 20MHz channels is common to avoid interfering with neighbouring APs. This does indeed lead to each radio having a usable throughput of ~50Mbps, so 100Mbps for the AP. I do also believe that the 600Mbps is only achieved with the 2.4GHz radio using a 40MHz channel that uses nearly all available 2.4GHz channels...
Modern APs do perform much better, but if I were the company I'd be buying docking stations and using the wired ethernet if possible. If that isn't possible buying a current WiFi6 AP model would help as the laptops should support this and you would get a lot better performance.
The MCS11 data rates of the MR56 are 1.1Gbps for 2.4GHz and 4.8Gbps for the 5GHz radio. Comparing this to the 300Mbps of each radio in the MR18 and limiting to 20Mhz gets you to 0.57Gbps and 1.1Gbps, then extrapolating the limitations and inefficiencies would lead to throughput of about 200Mbps for 2.4GHz and 400Mbps for 5Ghz which would be about right with testing I have done, though you may get about double of that due to the additional streams available which a single client cannot use.
I'd estimate that a single MR56 with 20Mhz channels would give you 600Mbps-1Gbps reliable throughput with 30 clients connected to each radio though the safer bet would be nearer the lower end of the range. Remember that other devices; phones, watches, printers etc. may well also connect to the WiFi and you want to size for no more than 30 devices per radio. More will work, but efficiency will be even worse.
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