So I’ve got my FreePBX setup with softphones for now… if I decide to say, add a couple 4-line phones, does each line belong to a separate extension or do I assign, say, line 1 on both phones to the same extension? In other words, do I need 8 extensions in Asterisk or do I just 4?
You should stop thinking of a PBX as a key system, my guess is you are trying to build one, look into hinting, BLF and SLA if your phone supports them.