Having A Ceremony
When you are planning on having a ceremony a lot of brides and grooms pick the same number of bridesmaids and groomsmen so the groomsmen can escort the bridesmaids. If you are not having a ceremony then you don't need to have the same number of bridesmaids and groomsmen.
Choosing your Bridesmaids
You should automatically include your sisters. If you don't have sisters then your best friends can fill the void. If the groom has sisters you should include them, as well as your brother's sisters. You're all going to be family once you say those two words- "I do." Having your future sister-in-law can be challenging for some, especially if your relationship is rocky, as was the case in my experience. Nonetheless you should include them as a bridesmaid. This may help mend the relationship, but if nothing else will help to minimize drama. So my advice, no matter how much the sister-in-law doesn't like you still ask them to be your bridesmaid. If there's going to end up be drama by not selecting someone as a bridesmaid, it's better to avoid it by just having more, trust me. Also it may be nice to include those friends or cousins who included you in there's. Girls think about those things and take notice. However if you've lost contact or aren't as good as friend's then obviously you don't need to worry about it. If a potential bridesmaid lives far away or has a schedule that makes them "hit or miss" it may be best not to include them. You need bridesmaids that can give you a strong commitment to be there.
Choosing your Groomsmen
Obviously include your brother's. If you don't have brother's then your best friends and if the bride has brother's include them. You should also include brother-in-laws on both sides. Whether they are the bride's brother-in-laws or your brother-in-laws.
I recently attended a wedding where the bride and groom included all their family and still had their friends as well. They ended up having a total of nine bridesmaids and groomsmen and it looked awesome for pictures. There is no rule on how many bridesmaids and groomsmen to have but if you have at least four of each, it looks good for pictures: the more the merrier. Now you obviously can't have everyone be a bridesmaid and groomsmen but at least put your immediate family up their first before you think of friends, cousins etc.
Maid of Honor and Best Man
Usually your sister is your Maid of Honor. If you have two sister's then pick the one you're closest to, or switch off so that each of you have a chance to be a Maid of Honor. If you have no sister's then pick your closest girlfriend. If you don't want to have drama then you don't have to have a Maid of Honor unless you are doing a ceremony then you should.
Your Best Man is typically the groom's brother. If you have two brother's like my fiancee did, he picked his brother right below him that he is closer with. There were no hard feelings because his youngest brother was still a groomsman. Again, if you don't have a brother then have your closest bro be your Best Man.
Flower Girl/Ring Bearer
You only need a Flower Girl and Ring Bearer if you are having a ceremony. You usually have a niece, nephew, or very young brother or sister to be the Flower Girl and the Ring Bearer. You only need at least one of each, but you can have a few.