Number of Installments is not editable even if the field "Employee can edit no of installment" is set to Yes.
Employee Central: Advances
This is a system limitation in new People Profile (PP3) and is known to engineering development team.
We still do not know any fix date for this issue. Once it is fixed, it'll be included in the release notes. Please note that it works fine in V12.
ECT-66142, advances, eligibility , KBA , LOD-SF-EC-AAD , Advances & Deductions , Problem