Summarize the following:
Even if you’re not sure on the specific figure, you can determine roughly what size of wedding you want. This will help you get quotes from caterers and venues while allowing you to figure out the specific number later.  Some people decide just to invite immediate family members and a few close friends. A small wedding is generally around 50-75 people.  The average wedding is around 150 people.  A large wedding is 200 people or more. The easiest way to decide how many people to invite is to look at how many people you can afford to invite. The more people you invite, the more expensive the reception and ceremony will be.  If your parents are chipping in, find out how much each couple is contributing. Factor that into how much you and your partner can afford to spend.  Get a few quotes from local caterers to see how much the reception may cost per person. Remember to factor in drinks and cake as well. Catering should take up about 25% of your budget. The number of people you can invite may be decided by where you have your wedding. If you have your heart set on a particular venue, ask them how many people they can hold. Do not invite more than this number, or else you may find yourself scrambling to fit everyone. Both you and your partner will want to invite important family and friends. Furthermore, both sets of parents may have ideas about who should come. A good way to fairly decide is to divide up the guest list amongst you, your partner, and both sets of parents. Some ways you can decide:  Divide the list in thirds. One third for your guests, one third for your partner’s guests, and one third for mutual friends.  Divide the list in fourths: one fourth for your guests, one fourth for your partner’s guests, one fourth for your parents’ guests, and one fourth for your partner’s parents guests. One half of the guest list is for you and your partner’s friends. Each set of parents receives one-fourth of the guest list. While traditionally it is advised that both sides of the family invite the same number of guests, you should divide the guest list based on your circumstances and preferences.

Summary:
Determine a ballpark figure. Consider your budget. Find out the size of the venue. Divide the guest list.