Use the following RegEx pattern for your RegularExpressionValidator:
(\b[a-z0-9]+\b.*){5,}
This pattern will match if the input has 5 words or more, it will fail if it is under 5 words. Of course, you'll have to do something a little bit different if you also want to verify that each word has a minimum length as well.
Example:
"a sample of a match" as an input string will pass this pattern.
"this does not pass" as an input string will fail this pattern.
Notes:
Single letter words will be valid with this RegEx pattern.
If you want to change the min count, just change the "5" in the last bracket set to your desired min. word count.
Modification to Pattern:
A simple modification of the above RegEx pattern for your RegularExpressionValidator:
(\b[a-z0-9]{3,}\b.*){5,}
This example will give you a minum word count of five, and each word must be at least 3 characters in length.
This one will not only give you your desired min. word count, but it will also validate that each word has a minumum length as well.
Notes:
Change the "{3,}" to your required min. word length
Change the "{5,}" to your required min. word count.
Hope this helps...