Rec'd a letter from Co. Child Support saying that I am being sued by the State of Wisconsin. My husband and I are separated, but not filed legally. What form am I supposed to complete within 20 days?

Call the county Child Support agency and ask Is your husband the custodial parent? For the state to sue you for child support, he would have to be the custodial parent and have filed for state support.

I'm not sure whether, when she wrote "Co. Child Support", whether she meant "county" or perhaps "Colorado". Normally child support agencies are on the state, not county level. At any rate, if OP is being sued by Wisconsin, she should be served with a court summons, not just a letter from a child support agency.

If this is an administrative action by a child support agency (rather than a lawsuit filed in court), an informal letter contesting the child support will probably do, but in any event, as ScottGem suggests, she can call and ask if there is any particular form which should be used.