I’m honestly wondering since study seem to primarily include Android users. It seems like the only main statistics they showed about notifications was for the most popular ones (including about eight specifically). It feels strongly plausible that 5,000 notifications a day could be coming from eight different apps.

Also, given how a lot of Android notifications are used for things like background updates, or also things like media controls (which are technically treated as notifications). This would also add a bit of plausibility to that metric. Especially in the case of a more digitally savvy user.