How the space from extracted teeth is shared out depends on how much room the remaining teeth in the arch need and on how far off the bite is. For example, if the front teeth clearly need a lot of room to line up, the whole space created by the extractions will probably be taken up by the front teeth alone. If premolars are taken out to correct the bite, the space may be closed by the front teeth, by the front and back teeth equally, or entirely by the back teeth, depending on what the bite needs. Planning how the space will be closed is one of the orthodontist's most important tasks.
This answer is for general information. Your own case needs an examination by an orthodontist.