this can be done using custom code. Basically, you can get the list item count and then update the field of the other list.
var ctx = SP.ClientContext.get_current();
this.list = ctx.get_web().get_lists().getByTitle("MyList");
ctx.load(list);
ctx.executeQueryAsync(Function.createDelegate(this, OnSuccess),
Function.createDelegate(this, OnFail));
function onSuccess(sender, args)
{
var count = this.list.ItemCount;
//now you can use count to update the column on the other list
}
To update a list item, you can do something like that:
var siteUrl = '/sites/MySiteCollection';
function updateListItem(count) {
var clientContext = new SP.ClientContext(siteUrl);
var oList = clientContext.get_web().get_lists().getByTitle('MyOtherList');
this.oListItem = oList.getItemById(3);
oListItem.set_item('ListCountColumn', Count);
oListItem.update();
clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}
function onQuerySucceeded() {
alert('Item updated!');
}
function onQueryFailed(sender, args) {
alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}