15 minutes = 900 seconds. 20000 Nodes / 900 sec = 9550
I you configure the NRF24 to the Maximum of RF24_2MBPS you can transmit - in theory - 2.000.000 MBit / sec = 250.000 Bytes / sec.
250000 Bytes/Sec / 32 Byte Payload size = 7812 payloads/sec. Thats less that the requiered 9550.
And if you must concentrate a lot of Clients on repeater nodes. you are totally lost. Forget it.