But if it's a different URL for each recipient then google will fetch each individual URL, for each email. So every URL will be opened exactly once, by the goog

But if it's a different URL for each recipient then google will fetch each individual URL, for each email. So every URL will be opened exactly once, by the google fetch that happens when the google server receives the email. The user will fetch the pixel from google cache. So how can you track open rates by pixel download when 100% of pixels are downloaded just one time? It probably depends on email clients. If a desktop client fetches email direct from smtp then it will probably also fetch the pixel directly. So tracking all downloads after the first (by google) may yield some signal, albeit not for gmail web users.