At level 5 it hits twice and at level 10 it hits three times. If you have the target hexed that’s 3 extra d6. If you have agonizing blast it’s your charisma modifier 3 times. It’s generally the best move in combat for a warlock to cast every turn which is why they have limited spell slots (although they do get them all back on a short rest). Also, depending on your dm, you may be able to apply the knock back 3 times for some considerable shenanigans.
This is actually very insightful. You may want to consider war caster so you can use eldritch blast in combination with sentinel. It's a bit OP as and creature that moves within range will take 3 elditch blasts and gets knocked back 3 times, depending on your DM's decision. You give up a lot of charisma by taking 2 feats instead of +2 charisma, but it is worth taking imo.