How many shades of blue do you need?

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body {
      margin:0
    }
  </style>
</head>
<body>
  <script>
    const width = 960;
    const height = 1000;
    const leftEdge = 25;
        
    const minN = 2;
    const maxN = 40;
    
    const svg = d3.select('body').append('svg')
      .attr('width', width)
      .attr('height', height)
      .attr('shape-rendering', 'crispEdges')
    
    const colors = [
      '#CCE3F2',
      '#99C7E4',
      '#66AAD7',
      '#338EC9',
      '#0072BC',
      '#00568D',
      '#18375F'
    ];
    
    const scale = d3.scaleLinear()
      .domain(d3.range(colors.length).map(d => d / (colors.length - 1)))
      .range(colors);
    
    const xScale = d3.scaleBand()
      .range([leftEdge, width]);
    
    const yScale = d3.scaleBand()
      .domain(d3.range(minN, maxN + 1))
      .range([0, height]);
    
    const data = [];
    yScale.domain().forEach(y => {
      xScale.domain(d3.range(y));
      xScale.domain().forEach(x => {
        data.push({
          x: xScale(x),
          y: yScale(y),
          width: Math.ceil(xScale.bandwidth()),
          color: scale(x / (y - 1))
        });
      });
    });
    
    svg.selectAll('rect').data(data)
      .enter().append('rect')
        .attr('x', d => d.x)
        .attr('y', d => d.y)
        .attr('width', d => d.width)
        .attr('height', yScale.bandwidth())
        .attr('fill', d => d.color);
        
    svg.selectAll('text').data(yScale.domain())
      .enter().append('text')
        .attr('x', leftEdge - 4)
        .attr('y', yScale)
        .attr('dy', '1.2em')
        .attr('fill-opacity', 0.26)
        .style('font-family', 'sans-serif')
        .style('text-anchor', 'end')
        .text(d => d);
  </script>
</body>

https://d3js.org/d3.v4.min.js