If you are curious about the caffeine in the hojicha you brew at home, public sources give the amount alongside the extraction conditions. Start by asking how the tea behind that figure was brewed.
20 mg per 100 mL, and the conditions behind it
The Ministry of Agriculture, Forestry and Fisheries lists hojicha infusion at 0.02 g per 100 mL, or 20 mg per 100 mL. The stated method uses 15 g of tea, 650 mL of water at 90°C, and 0.5 minutes.
The tea figures in this table are based on the Standard Tables of Food Composition in Japan 2020 (eighth revised edition). If the amount of tea, water volume, temperature or time changes, the same value cannot simply be applied unchanged.
Differences in brewing, differences between people
The Food Safety Commission of Japan explains that caffeine content varies with the type and amount of ingredients and the extraction method, so no single general value can be given.
The commission also explains that sensitivity to caffeine differs greatly between individuals, making its health effects difficult to assess accurately. For this reason, no acceptable daily intake (ADI) has been set internationally. It also introduces intake guidance from overseas bodies. The absence of an ADI does not mean that there is no guidance on intake.
This figure alone cannot tell you how many cups to drink
Both the caffeine content of brewed tea and individual sensitivity vary, so this figure alone cannot establish a suitable number of cups to drink.
If you are pregnant, choosing a drink for a child, or concerned about your own sensitivity to caffeine, please consult official guidance, such as that from the Food Safety Commission of Japan.