Like many aspects of wedding planning, there were once rather rigid guidelines that ruled the how and the who of weddings. Some experts used a formula that said there should be a bridesmaid and usher for every 45-50 guests. Fortunately, that has long been abandoned.
Bridesmaid numbers depend on who is close to the bride and who she wants to stand up for her. Likewise the groom is freer to select who and how many of his friends to include. But when selecting attendants, remember an important guideline - how many can you afford to have? Every attendant is entitled to bring a guest so realize that every one you have means two for the reception per plate charge.
Once you've decided on a number, the bride and the groom should prepare a priority list of attendants.
You'll need to consider each other's families in creating the selection list. A bride may wish to include the top tier of her sorority sisters leaving no room for the groom's sisters or the groom may wish to have a group of fraternity brothers plus his own brothers. Once a number of attendants is agreed upon, something has to give.
What happens if the bride has eight friends she can't live without in her wedding party, but the groom - an only child has only 4 close buds? If your budget can handle the expense, there are always ways to handle an unbalanced wedding party. For the processional, one groomsman can escort two bridesmaids - one on each arm. Or if symmetry is important, forget about male/female and just go by the numbers. You have 12 attendants total and there will be 6 on each side of the bride and groom - 2 guys and 4 girls on each side. This works especially well in wedding photos. The result is balanced with color and diversity on each side of the bridal couple.