"Supplemental interrogatories" are a second set of questions that are generated by opposing counsel in any civil litigation. Your husband must provide answers to this second set of questions just as he did the first set of interrogatories. With respect to the request for him to provide your social security number, I suggest that he simply leave this answer blank. If the other side raises the issue, your husband or his attorney can always raise an objection to the question, and let the judge make the call as to whether he must provide this information.
Parties to litigation are allowed a given number of interrogatories in accordance with court rules, which vary from court to court. So, for example, if the court allows 30 interrogatories and the first set of interrogatories included only 25 interrogatories, then a second set of "supplemental interrogatories" can be sent with up to 5 more questions. If they try to push 10 on you, an objection is appropriate.
As to the SS#, I never answer this, but you need to object now, within the time provided by the court rules or you could be deemed to have waived the objection. Do not wait to object. I always object to SS# as "unduly burdensome" in the context of rampant identity theft, disclosure of personal identification information on a semi public document is improper. Many courts actually have rules requiring redaction of such personal information.
Finally, you are not doing a custody case without an experienced attorney if you are serious about the case and so you must consult with your attorney to make sure that the interrogatories are not an abuse of the discovery process. In the alternative, find and read all of your court's rules of civil procedure.
This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.
Generally, in Illinois a party may not serve more than thirty (30) interrogatories, including sub-parts, without leave of court granted on good cause shown after written motion. The motion for leave to serve interrogatories in excess of thirty (30) must set forth the proposed questions and the reasons establishing good cause. IL Supreme Court R. 213(c).
Supplemental interrogatories can be propounded to request additional or supplemental information about the dispute. Leave of court is usually required.
The party served with interrogatories must answer or object to each question. IL Supreme Court R. 213(d).
If you object to the social security number request, you should reference a legal reason why. Each answer or objection must be immediately preceded by a full restatement of the interrogatory to which it responds. IL Supreme Court R. 213(d).