Number seq is getting increased if I just open and close a form without saving it

You have to implement all the calls to the number sequence framework as documented here.

Also you have to mark the number sequence as "Continuous" if you want to reuse unused numbers.

Method in form:

public NumberSeqFormHandler numberSeqFormHandler()
{
    if (!numberSeqFormHandler)
    {
        numberSeqFormHandler = NumberSeqFormHandler::newForm(
            FCMParameters::numRefWorkOrderNum().NumberSequenceId,
             element,
             FCMWorkOrders_DS,
             fieldnum(FCMWorkOrders, WorkOrderNum)); 
    }
    return numberSeqFormHandler;
}

Methods in form datasource:

public void create(boolean _append = false)
{
    element.numberSeqFormHandler().formMethodDataSourceCreatePre();
    super(_append);
    element.numberSeqFormHandler().formMethodDataSourceCreate();
}

public void delete()
{
    element.numberSeqFormHandler().formMethodDataSourceDelete();
    super();
}

public void write()
{
    super();
    element.numberSeqFormHandler().formMethodDataSourceWrite();
}