“America consumes 8.5 million flannel shirts a year” on an exhibition label as a hard fact.
I'd phrase it:
ESTIMATED 7–10 MILLION PLAID/CHECK FLANNEL SHIRTS ARE SOLD IN THE UNITED STATES EACH YEAR.
And underneath:
Estimate derived from US apparel customs classifications, import shipment records and independent flannel-market research. Flannel is not separately identified in official US trade statistics.
That actually makes the artifact stronger because you're showing the methodology and uncertainty, rather than manufacturing false precision.
And there is an even better next step
I think we can go one level deeper and reconstruct the actual American flannel supply chain:
How many are made in Bangladesh / China / Vietnam / Pakistan / India → which American companies import them → approximate quantities → which brands sell the most → how many end up in thrift → how much is exported → and specifically how much eventually reaches Africa.
That would turn your flannel shirt from simply an interesting garment into a quantifiable object tracing global trade, consumption and waste — which fits Material Culture extremely well.
Using the estimate we just established:
≈ 8.2–12 million plaid/check flannel shirts per year
A useful central estimate would be approximately 10 million shirts per year.
That works out to roughly:
For your project, I would use “approximately 10 million plaid/check flannel shirts a year” rather than the wider range when you need a clean headline figure — but label it as an estimate, not an official statistic.