Can I have an element with an ID that starts with a number?

Can I have a div with id as number?

Yes you can, but selecting/styling it with a CSS selector will be a pain.

id values that consist solely of digits are perfectly valid in HTML; anything but a space is okay. And although earlier HTML specs were more restrictive (ref, ref), requiring a small set of chars and starting with a letter, browsers never cared, which is a big part of why the HTML5 specification opens things up.

If you're going to use those ids with CSS selectors (e.g, style them with CSS, or locate them with querySelector, querySelectorAll, or a library like jQuery that uses CSS selectors), be aware that it can be a pain and you're probably better off staring the id with a letter, because you can't use an id starting with a digit in a CSS id selector literally; you have to escape it. (For instance, #12 is an invalid CSS selector; you have to write it #\31\32.) For that reason, it's simpler to start it with a letter if you're going to use it with CSS selectors.

Those links above in a list for clarity:

HTML5 - The ID Attribute
HTML4 - The ID Attribute and ID and NAME tokens
CSS 2.1 rules for IDs

Below is an example using a div with the id "12" and doing things with it three ways:

With CSS
With JavaScript via document.getElementById
With JavaScript via document.querySelector (on browsers that support it)

It works on every browser I've ever thrown at it (see list below the code). Live Example:

"use strict";

document.getElementById("12").style.border = "2px solid black";
if (document.querySelector) {
    document.querySelector("#\\31\\32").style.fontStyle = "italic";
    display("The font style is set using JavaScript with <code>document.querySelector</code>:");
    display("document.querySelector(\"#\\\\31\\\\32\").style.fontStyle = \"italic\";", "pre");
} else {
    display("(This browser doesn't support <code>document.querySelector</code>, so we couldn't try that.)");
}

function display(msg, tag) {
    var elm = document.createElement(tag || 'p');
    elm.innerHTML = String(msg);
    document.body.appendChild(elm);
}

#\31\32 {
    background: #0bf;
}
pre {
    border: 1px solid #aaa;
    background: #eee;
}

<div id="12">This div is: <code>&lt;div id="12">...&lt;/div></code>
</div>
<p>In the above:</p>
<p>The background is set using CSS:</p>
<pre>#\31\32 {
    background: #0bf;
}</pre>
<p>(31 is the character code for 1 in hex; 32 is the character code for 2 in hex. You introduce those hex character sequences with the backslash, <a href="http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">see the CSS spec</a>.)</p>
<p>The border is set from JavaScript using <code>document.getElementById</code>:</p>
<pre>document.getElementById("12").style.border = "2px solid black";</pre>

I've never seen the above fail in a browser. Here's a subset of the browsers I've seen it work in:

Chrome 26, 34, 39
IE6, IE8, IE9, IE10, IE11
Firefox 3.6, 20, 29
IE10 (Mobile)
Safari iOS 3.1.2, iOS 7
Android 2.3.6, 4.2
Opera 10.62, 12.15, 20
Konquerer 4.7.4

But again: If you're going to use CSS selectors with the element, it's probably best to start it with a letter; selectors like #\31\32 are pretty tricky to read.

From the HTML 5 specs...

The id attribute specifies its element's unique identifier (ID). [DOM]

The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters.

There are no other restrictions on what form an ID can take; in particular, IDs can consist of just digits, start with a digit, start with an underscore, consist of just punctuation, etc.

An element's unique identifier can be used for a variety of purposes, most notably as a way to link to specific parts of a document using fragment identifiers, as a way to target an element when scripting, and as a way to style a specific element from CSS.

Identifiers are opaque strings. Particular meanings should not be derived from the value of the id attribute.

So... yes :)

From the HTML 4.01 specs...

ID must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".").

So... no :(

You can also select that type of id(though it is definitely not the best practice to create such an id that starts with a number) by doing the following:

document.querySelector('div[id="12"]'); //or 
document.querySelectorAll('div[id="12"]'); //if you have multiple elements with equal ID.

From a maintainability standpoint this is a bad idea. ID's should be at least somewhat descriptive of what they represent. Prefix it with something meaningful to be compliant with what others have already answered with. For example: