How many concurrent connections I can get with BL654 loaded with AT Commands interface app code?

The default maximum numbers of BLE connection allowed with AT Commands is 8.

Assuming you are working with the AT Command interface application version 4.04 and Library version 5.02 or newer, you can reach until 16 concurrent BLE connections through non-VSP mode:

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To do so, you'll first need to set up your BL654 to non-VSP mode via S Register 100 (Start-up Flags) then increase the maximum allowable concurrent connections within S Register 126 (Max Connections as Master/Central) as follows:

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You'll then be able to connect to until 15 BLE devices as master (AT+LCON) plus 1 as slave (AT+LADV) as follows:

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Please note that multiple connections means sharing bandwidth where latency and throughput will be significantly impacted.