。
There are many things we need to discuss.
Literal
Discuss [must-ねばならない] things [subject-が] many exist.
~ねばならない is the literary equivalent of ~なければならない, using the classical negative conditional ~ねば. 話し合う is a compound verb: 話す + 合う — the ~合う suffix adds mutuality, turning 'speak' into 'discuss together.' The whole obligation clause modifies こと as a relative clause.