How to select first N elements with jquery?

Actually, you don't need any conditional statement, you can do it with a single selector:

$('ul li:first').mouseenter(function() {
});

If you want to further "filter" your <li> nodes, be more specific, like

$('ul li.navahead:first').mouseenter(function() {
});

update:

To answer your updated question:

$('ul li[class^=tab]').mouseenter(function() {
});

^= will select all classnames which begin with a given string ("tab" in this case)

Use .slice(startIndex, endIndex)

$("div").slice(0, 2).remove();

<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>


<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

You don't need an if - you can use the jQuery :lt() selector

$('ul li:lt(3)').mouseenter(function(){});

Note that the index is zero-based, so the first three are 0, 1 and 2

http://api.jquery.com/lt-selector/

Update July 2020:

As of jQuery 3.4, the :lt pseudo-class is deprecated. Remove it from your selectors and filter the results later using .slice(). For example, :lt(3) can be replaced with a call to .slice(0,3)

You don't need "if". Use selectors

$("ul li:first")