Do I have too many regexes in my validation logic?
For one, I wouldn't worry about the number of regexps here. There's nothing terribly complex going on here. I'd say regular expressions are the right tools for this job, so no reason to change that.

I do have some other comments though:


I'd advice you to always use English in your code, so phoneNumber, formattedPhoneNumber etc.. I'm not a native English-speaker myself, but I never write code in any other language. Just for consistency.
You can combine the first step of sanitizing the input and checking if there is any. To avoid indenting the code, you can also just return early.
While you're at it, you may want to simplify the regexp a bit. You basically just want to replace everything that isn't a digit or a plus sign:

var input = executioncontext.getEventSource().getValue();
var phoneNumber = String(input).replace(/[^\d+]/g, '');
if(!phoneNumber) return;

The large regexp may be simplified a bit (and be made more consistent by always using \d in place of 0-9). I'd write it as follows, and use match instead of test

var match = phoneNumber.match(/^(?:\+32|0032)?0?([1-9]\d{7}|4\d{8})$/);
if(!match) return; // again we can return if the number wasn't matched



You've already got the formatting part handled well. I've changed it a little, but nothing major. I've swapped out \d for simply . (since we know we're only dealing with digits at this point) but that's not necessary - \d would do equally well.

function acm_mscrm2011_psa_FormatPhoneNumber(executioncontext) {
  var input, phoneNumber, match, formattedNumber;

  input = executioncontext.getEventSource().getValue();
  phoneNumber = String(input).replace(/[^\d+]/g, '');

  if(!phoneNumber) return;

  match = phoneNumber.match(/^(?:\+32|0032)?0?([1-9]\d{7}|4\d{8})$/);

  if(!match) return;

  if(match[1].length === 8) {
    formattedNumber = match[1].replace(/(.)(...)(..)(..)/, "+32 $1 $2 $3 $4");
  } else {
    formattedNumber = match[1].replace(/(...)(..)(..)(..)/, "+32 $1 $2 $3 $4");
  }

  executioncontext.getEventSource().setValue(formattedNumber);
}


You could also combine a couple of steps, and do

function acm_mscrm2011_psa_FormatPhoneNumber(executioncontext) {
  var input, match, formattedNumber;

  input = executioncontext.getEventSource().getValue();
  match = String(input)
    .replace(/[^\d+]/g, '')
    .match(/^(?:\+32|0032)?0?([1-9]\d{7}|4\d{8})$/);

  if(!match) return;

  if(match[1].length === 8) {
    formattedNumber = match[1].replace(/(.)(...)(..)(..)/, "+32 $1 $2 $3 $4");
  } else {
    formattedNumber = match[1].replace(/(...)(..)(..)(..)/, "+32 $1 $2 $3 $4");
  }

  executioncontext.getEventSource().setValue(formattedNumber);
}