billboard vision level, questions about tiles

I am using billboards in my application for the quadtree level, just have a question, how do I know the amount of tiles are loaded by level?

var = viewer.scene.globe._surface._tilesToRender tiles;

I'm working on the clock event cesium
viewer.clock.onTick.addEventListener (function () {// check levels and applies});

(Basically verifies level and puts true view on the billboard, if you change your level does the same thing)

if I knew how many tiles would be charged per level, without having to use timeouts and intervals and use own methods, I believe it would be better ...
because I am using data from the database and be always comparing tiles is no loss of performance ... is there any method or calculation that I can know the amount of tiles that will be rendered in the scene?

var tiles = viewer.scene.globe._surface._tilesToRender;