You can SEND out requests to as many neighbours as you have. You can only RECEIVE 20. You absolutely should send the request again to all of your neighbours as you need 20 back to complete all 3 parts (3, 7, 10). If you don't, then all you miss out on is a mermaid chest. If it's a daily (and we don't know that yet) request then you'll be ahead tomorrow!
I've been sitting on 19 all day but there is still many hours left till the goal expires.