I have four best friends and a sister, and I am torn about who to ask to be in the wedding! It's not big enough to have them all (only 100 wedding guests)!
Scrap the X amount of attendants to X amount of guests rule. It's ridiculous. The only time it makes any sense is if you have ten guests and want eight bridesmaids (that would look a bit odd). In your case, the proportion of five attendants to 100 guests is perfectly appropriate. And if these women are your best friends, that's reason enough to have them all in your wedding party. Don't hurt any of your nearest and dearest needlessly. Give them all an honored role in your wedding.
See More: Choosing Wedding Attendants , Your Bridal Party