How to produce given number of quad in math

\quad sets a length of 1em, with multiples X of it being of length Xem. So we can define \myquad[<num>] with <num> being optional (default of 1), to produce a space of <num>em:

\documentclass{article}

\newcommand{\myquad}[1][1]{\hspace*{#1em}\ignorespaces}

\begin{document}

x \quad x \par
x \myquad[1] x

x \quad\quad\qquad x \par
x \myquad[4] x

x \qquad\qquad\qquad\qquad\qquad x \par
x \myquad[10] x

\end{document}

We use \hspace* to insert a space regardless of whether you're at the start, middle or end of a rule.

Here is version using a loop:

\documentclass{article}
\usepackage{pgffor}
\newcommand\Quad[1][1]{\foreach \Quaddy in {1,...,#1}{\quad}\ignorespaces}

\begin{document}

  x \quad x \par
  x \Quad[1] x

  x \quad\quad\qquad x \par
  x \Quad[4] x

  x \qquad\qquad\qquad\qquad\qquad x \par
  x \Quad[10] x

\end{document}

with output:

Personally, I prefer the more direct approach in Werner's answer.

You can look up how \quad and \qquad are defined, either in the LaTeX source or by asking LaTeX itself:

This is pdfTeX, Version 3.14159265-2.6-1.40.16 (TeX Live 2015/Debian) (preloaded format=latex)
 restricted \write18 enabled.
**\show\quad
entering extended mode
LaTeX2e <2016/02/01>
Babel <3.9q> and hyphenation patterns for 81 language(s) loaded.
> \quad=macro:
->\hskip 1em\relax .
 \show\quad

?        

*\show\qquad
> \qquad=macro:
->\hskip 2em\relax .
 \show\qquad

? 

*
! Emergency stop.
 \show\qquad

No pages of output.
Transcript written on texput.log.

In many cases this will place you in a maze of twisty commands, all subtly different, but here it's pretty straightforward. A quad is a horizontal space of 1em (the nominal width of a capital M in the current font).

\newcommand{\myquad}[1]{\hskip #1 em\relax}

or in a more LaTeXish way

\newcommand{\myquad}[1]{\hspace{#1 em}}