If you can already do 7 pull-ups, going for 12 it will not be much of a problem for you. You can try a few different ways to improve your number.
-Since 7 is the number of pull-ups you can do right now, start doing pyramids of 1-2-3-4-5-6-7-6-5-4-3-2-1 two to three times a week. Once you can do more than 7 pull-ups, for example 8 pull-ups then increase your number of the pyramid to 8 (1-8 and then back down to 1). You can rest as much as you need between the sets since the objective here is to complete the next number of pull-ups and not how fast you can do them.
-Another option for you is to add weight while you are doing your pull-ups. This can be done either with the assistance of a weighted belt as Jeremy suggested or wearing a weighted vest or holding a dumbbell/kettlebell with your feet or wearing a backpack with weights in it. I would start with 10lbs. until you can perform at least 5 rounds of 1-5 reps (ascending pyramid 1-2-3-4-5, then start the second round from 1). Once you do this then add some more weight and do it all over again.
-As Jeremy also suggested, one more re option for you would be to use the assistance of a band while you are doing your pull-ups to increase the muscular endurance of those muscles needed to help you perform your pull-ups. When you use this method, set a number of repetitions to do on a given workout and don’t worry how long it takes you. You can start with 50 and then increase the number to a higher one. You can take as many breaks as you need as long as you complete the targeted number.
I have used these techniques with many of my clients and most of them are now in the range of 15-25 pull-ups that they can do in a row. There a different ways to go about this, so you need to see which one works best for you (or you can incorporate all of the above into your workout plan).