If you need one line for a memo or slide:
Estimated aggregate audience size: ~550M monthly visits, with a reasonable scenario range of 400M to 800M.
The database contains 108 outlets, and the mix includes a few very large publishers plus a long tail of mid-size and niche outlets.[1]
The benchmark points I could verify already span a huge range:
That means the portfolio is highly skewed, so a scenario model is more useful than pretending there is one exact number.
If you want a single uncertainty label, I would use:
That is wide enough to be honest, but still useful for strategy work.