textbox to have limit value

 i have 3 textboxes  
<asp:TextBox ID="txt_12hours" runat="server"></asp:TextBox> <asp:TextBox ID="txt_24hours" runat="server"></asp:TextBox> <asp:TextBox ID="txt_minutes" runat="server"></asp:TextBox> how can i limit the textbox....
eg..
txt_12hours = user can only type max value 12 hours, so user can't type 22, 13 ...etc..
txt_24hours = user can only type 0-23, no alphatxt_ ...