The commands <b>xtickmarks</b> and <b>ytickmarks</b> are probably just
what you are looking for.  Try these.

<p>
>plot(sin(x), x=-2*Pi..2*Pi, xtickmarks=[-6.28=`-2Pi`,-3.14=`-Pi`, 0=`0`,3.14=`Pi`,6.28=`Pi`],
  ytickmarks=[-1=`-1`,1=`1`]);
<br>
<br>
> J := (n,x) -> sqrt(Pi/(2*x))*BesselJ(n+1/2,x):
<br>
> plot (J(0,x),x=0..20, `J(0,x)`=-1..1, 
         xtickmarks=4,ytickmarks=3,
         title=`Spherical Bessel function J(0,x) of the first kind`,
         titlefont=[HELVETICA,BOLDOBLIQUE,14] );

<p> Perhaps the behaviour of xtickmarks and ytickmarks here is not quite what you
expected.  From Maple help, we read that 
<b>xtickmarks=n</b>
Indicates that a <em>reasonable</em> number of points no less than n 
should be marked along the horizontal axis; n must be a positive integer or a list. 
If n is a list, then the list of values is used to mark the axis; the corresponding option 
ytickmarks=n to specify the minimum number of divisions along the vertical axis, or a list 
of values used to mark the vertical axis.