You can declare as many buttons you want and assign 1 onclick function :)
Tested for 4 buttons
<asp:Button ID="btn1" runat="server" Text="btn1" onclick="btn1_Click" />
<asp:Button ID="Button1" runat="server" Text="btn1" onclick="btn1_Click" />
<asp:Button ID="Button2" runat="server" Text="btn1" onclick="btn1_Click" />
<asp:Button ID="Button3" runat="server" Text="btn1" onclick="btn1_Click" />