I think this can easily be done using a client script.
function onSubmit(){
//Set the mandatory checkbox variable names and total mandatory count here
var mandatoryVars = 'byod_airwatch,pcl,haiku,rover,software_other';
var mandatoryCount = 1;
var passed = forceMandatoryCheckboxes(mandatoryVars, mandatoryCount);
if(!passed){
//Abort the submit
alert('You must select at least ' + mandatoryCount + ' software.');
return false;
}
}
function forceMandatoryCheckboxes(mandatory, count){
//Split the mandatory variable names into an array
mandatory = mandatory.split(',');
var answer = false;
var varFound = false;
var numTrue = 0;
//Check each variable in the array
for(x=0;x<mandatory.length;x++){
//Check to see if variable exists
if(g_form.getControl(mandatory[x])){
varFound = true;
//Check to see if variable is set to 'true'
if(g_form.getValue(mandatory[x]) == 'true'){
numTrue ++;
//Exit the loop if we have reached required number of 'true'
if(numTrue >= count){
answer = true;
break;
}
}
}
}
//If we didn't find any of the variables allow the submit
if(varFound == false){
answer = true;
}
//Return true or false
return answer;
}
If you still want to use UI script, response from Paul in below link should give you the solution
