I used to travel a lot for business, and always carried 2 in my bag. You'd be surprised at how much clothing condenses in volume when the air is all squished out and then what little room it takes up in a suitcase.
Don't know if clothing for 5 would work for one bag, but I guess it all depends on the size of the suticase and what the clothing and sizes are

If you have clothing for 2 little kids that consist of shorts and t's, 5 days of that type of clothing in a space bag could condense down to just about nothing.
I've used space bags for about 5 years now and love 'em! I use them around th house to store things like comforters, pillows, etc. But my most frequent use was for travel.