Need advice for a project (multiple arduino connection)

Hi experts, just want to ask some advice the best solution possible regarding a project in mind.
I need to connect at least 25 arduino as slaves which consist of 3 buttons(menu) each. Each buttons will send data to the Master with the following format [from Mcu1 Data2(button2)]. I appreciate any input and advice regarding my project in mind. Please check my attachment.

Thanks in advance,
mbox