Seventy carries ideas of fullness and completeness — seventy elders helped Moses, seventy years marked the exile, and Jesus sent out seventy to proclaim the kingdom. It suggests a work carried to its full extent.
Jesus also told Peter to forgive not seven times but seventy times seven — using the number to mean forgiveness without keeping count, mercy beyond measure.
The Number Seventy in Scripture
After these things the LORD appointed other seventy also, and sent them two and two before his face into every city and place, whither he himself would come.