Just so you have all the info you need, just some FYIs.
That was true up until they revised the party about 6 months ago. It still is the case but only during peak times. Contrary to what's being posted that the summer months are going to be slow, Disney's own crowd level data is predicting otherwise, but what do they know. IE, the parties have a good chance of eventually selling out during your month of July, plus crowds will be higher at DHS due to TSL opening.
Again, it depends on the attendance number on your party night. Up until the peak of the Spring Break season and immediately after it ended, party attendance was somewhat low. However during busier Spring Breaks, it sold out almost every night. Whenever it is sold out, 210 guests are not able to sit at the viewing area.