Code

var layers = [
    [150, '#f28cb1'],
    [20, '#f1f075'],
    [0, '#51bbd6']
];
layers.forEach(function(layer, i) {
    map.addLayer({
        "id": "cluster-" + i,
        "source": "markers",
        "type": "circle",
        "paint": {
            "circle-color": layer[1],
            "circle-radius": 10
        },
        "filter": i === 0 ? [">=", "point_count", layer[0]] : ["all", [">=", "point_count", layer[0]],
            ["<", "point_count", layers[i - 1][0]]
        ]
    });
});
map.addLayer({
    "id": "cluster-count",
    "type": "symbol",
    "source": "markers",
    "layout": {
        "text-field": "{point_count}",
        "text-font": [
            "Arial Unicode MS Bold"
        ],
        "text-size": 12,

        "icon-image": "{marker-symbol}",
        "icon-size": 0.20
    },
    "paint": {
        "text-color": "white"
    }
});