Here's the basic outline for a transaction filter that checks quantities of further asset issuances:
function filtertransaction() {
    var tx = getfiltertransaction();
    for (j = 0; j < tx.vout.length; j++) {
        if (tx.vout[j].assets) {
            for (k = 0; k < tx.vout[j].assets.length; k++) {
                if (tx.vout[j].assets[k].type == "issuemore") {
                    var newqty = tx.vout[j].assets[k].qty;
                    var oldqty = getassetinfo(tx.vout[j].assets[k].name).issueqty;
                    if (oldqty + newqty > 0) return "New qty " + newqty + " old qty " + oldqty;
                }
            }
        }
    }
}
You'll need to replace the line with if (oldqty + newqty > 0) to reflect whatever rule you want to apply, and whatever error message you want the filter to return if the rule is violated.