gistfile1.txt

Input.cart.line_items.each do |line_item|
  product = line_item.variant.product
  max_qty_tag = product.tags.find { |tag| tag.start_with?('max_') }

  if max_qty_tag
    max_qty = max_qty_tag.gsub('max_', '').to_i
    reduce_by = line_item.quantity - max_qty
    line_item.split(take: reduce_by) if reduce_by > 0
  end
end

Output.cart = Input.cart

Shopify Code Editor Script
checks product tags for max_1, max_2, max_3 ... etc and only allowed that item to be purchased that many times 1, 2, 3 etc