The number of the homeless will reach 19 million by the end of this decade. > You use "will"( future tense) to indicate YOU ARE SURE that it is going to happen in the future.( your strong opinion on this fact) The number of the homeless will have reached 19 million by the end of this decade. > You use "will have" (future perfect tense) to indicate something YOU SEE BEING COMPLETED by that time in the future. By this time next year (future), I will have gone to 20 other countries > You are looking back at it from a future perspective and expect to see what you said earlier done)

Yes

The number of the homeless will reach 19 million by the end of this decade. This is a simple future tense. Future perfect tense WILL HAVE REACHED is used when two actions occur in the future. The previous action should be in future perfect tense and the later action should be in simple present tense. For instance, the number of the homeless will have reached 19 million by the time the year ends. Hope this could make some sense.