This is a big question. If your site averages 100 million page views per month, and you monitor 100% of that traffic, things can get pricey pretty fast – especially during traffic surges. (You know this already if you've tried other RUM tools.)
The number of page views needed depends on how much you drill down into the data. For example, if your site is fairly uniform – a single type of content, one main geographic region, no A/B testing – then a lower number of page views is probably sufficient. But if you have multiple types of content, target more than one country, are accessed from a variety of devices, and want to use LUX's A/B testing capabilities, you might need more page views in order to have enough data when drilling down.
How to determine the right sample rate
Most RUM vendors don't have data sampling built into their product. We do. You can set the sample rate across all pages under Settings -> LUX. If you want more fine-grained control (such as setting different sample rates for different countries, pages, devices, etc.) you can do that with the LUX API using
LUX.samplerate. (See Setting Your LUX Sample Rate for more details.) But what's the right number?
Setting your LUX sample rate starts by knowing the number of monthly page views for your site. For example, suppose your site gets ~100 million page views per month. If you buy a LUX plan that gives you up to 10 million pageviews per month, you would set the sample rate to 10%. (In fact, you might want to set it to 8% to allow some wiggle room for any spikes you might experience during the month and growth over the year.)
As you use the LUX dashboards, if you find that you're drilling down to where the number of page views is too low (e.g., less than 1000 per data point), then you might want to increase your budget by adding an additional bundle of 10M page views. Then with a budget of 20 million page views, you could set the sample rate to 20% (or 18% to be safe).
If you have any questions about calculating sample rate, let us know at firstname.lastname@example.org and we'll be happy to help.