I suggest AT LEAST 25 songs for cocktails. Cocktails is dependent on how long the pictures take sometimes for example. If it is raining or there are other unforeseen elements that get in the way this can sometimes take longer.
I suggest at least 40 songs for dinner. This of course depends on how many guests you have and what type of dinner you are having. Larger numbers take longer to serve. Plated meals generally take longer than buffet style dinners. If you are having a buffet style dinner with over 100 guests I would always inquire about a double-sided buffet. Double-sided buffets always go faster. For plated meals, unless you have a small group, under 100 guests, I would suggest at least 50 songs for dinner.
Brides and grooms often forget about table visitation. This generally happens after you have eaten. In a buffet situation, this can happen sooner than a plated dinner, which means you can get to dancing faster. The table visiting needs to happen before dancing because the success of the dance floor is dependent on how much time you spend on the dance floor. This is the time of the reception to put all your favorite songs. Sometimes the parents have a say in this and want calmer music and that is fine too. It is going to be background music so for the most part people will hardly recognize the type of music you are playing for dinner.
**NOTE: Dinner almost always goes longer than planned.
I suggest about 20 songs. Regardless if you are planning 15 -30 minutes for your pre-ceremony it is always good to have extra songs in case things get delayed. Be sure to pick more music than you need. It is difficult to get all the groomsmen, bridesmaids, parents, wedding officiant, photographer etc. to all be ready at the same time. So often, the ceremony starts a few minutes late. Don't add another level of stress by not having enough music to cover if things are running a little late.
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I suggest about 20 songs per hour. For two hours of dancing I recommend 40 songs. For 3 hours of dancing about 50 songs. Typically weddings end up with an average of 2 1/2 hours of dancing and get through about 40 - 45 songs.
Slow Dancing: I suggest about 10 songs MAX. This is dependent on your crowd. If you have a lot of couples and very few singles coming, then more slow songs might be nice. Otherwise, I normally suggest 1-2 slow songs an hour. A slow dance eliminates anyone from the dance floor that does not have someone to dance with so you don't want too many. If you have three hours of dancing time you will need about 6 slow songs. I suggest picking 10 slow songs, and put your favorite 6 slow songs first in the list.