Usually you get as many bread per video as you need to perform an attack. E.g. if you need 100 bread for an attack and have 40 bread left in the silo, you’ll get 60 more from the video to have then 100 and be ready to attack. If, however, you already had 96 before, you’ll only receive 4 more bread as that is all you need for the next attack. So, when you only need 4 more bread, waiting for a minute and collecting those from your farms might be the better option.
Not sure if there is a certain number where the offers are capped. Once I tried it out and was able to perform about 20 or so attacks in a row - all with free food from videos, so 20 videos - and I still got more offers. At some other time, I got no offers at all. Maybe it depends on day time / number of players online and the internet connection speed?
Also, since the last update, sometimes the button was clickable, but then after some seconds a message appeared saying “No offers available”, returning me to the screen with the “fill silo with gems” and “watch video” buttons, with the video button greyed out now. But when I pressed back to return to the enemy base preview screen, and then clicked on “attack” again (going again to the screen with “fill silo with gems” and “watch video” buttons), the “watch video” button was back in normal color and clickable again. Very strange!
Moreover, I noticed that sometimes I got gems instead of food from those videos… looks to me as if the mechanism has some bugs!