validate asp.net text field?

I have a text field asp:textbox which I must enter from 1 to 2 integers from a keyboard created with button type="button", I have not used asp:button because this keyboard and this textbox are inside a bootstrap modal , and if I used an asp:button it just restarted the page and closed the modal. Well the case is that I need to validate the textbox so that it only accepts a certain number of numbers.

I tried it with the validation tools that .net already has but it doesn't work for me, it doesn't let me write anything in the textbox with the buttons, so I removed it and tried it with jquery, but it only limits the space when I write with the keyboard of my pc, and if I write with the keyboard that I did, it does not limit me at all.

I hope I made myself understood. Thank you for reading.

<asp:textbox id="sem" runat="server" autocomplete="off"></asp:textbox>

<button id="b1" type="button">1</button>
<button id="b2" type="button">2</button>
<button id="b3" type="button">3</button>
<button id="b4" type="button">4</button>
<button id="b5" type="button">5</button>
<button id="b6" type="button" ...etc..

//Utilice esta función pero como ya dije, si escribo con el teclado que he 
hecho me deja escribir más de 2 números.

function validar(){
	sem=$('#head_sem').val();
	if (sem.length<2){
		return true;
	}
	else {
		alert('Maximo 2 caracteres');
		return false;
		
	}