Actually there is always a chance to get a net negative out of a harvest, it just gets smaller and smaller the larger your farm is. But on the other hand you continually find additional plants (especially corn and potatos in farm POIs) so even extreme bad luck never gets one really in trouble as the found plants can be made into seeds to fill up empty slots. In a recent game I had 8-10 plots for corn and potatoes each and I could supply 4 people with food. For all the other plants I only used around 4 plots each.
Sometimes I had to make all plants of one type into seeds for one or two harvests to keep them at 4. But it didn't matter because I could always cook different recipes when one of those plants was in reduced supply for a week. Even corn and potatoes could have a really bad harvest but I don't remember that I ever came into the situation that I didn't have enough plants to replant all pots. Or if it did happen it must have been so easy to correct that I forgot it did happen. I remember there were harvests where I just got a handful of corn or potatoes for cooking though.
Each harvest was different. In most harvests at least one of the plant types had a relatively bad harvest, sometimes even the worst case, say 0 out of 4. That meant I kept the whole harvest for seed production, i.e. produced 3 seeds and even kept the 1 remaining plant to bolster the next seed production. But this never had even a chance to go into a downward spiral because I also found seeds and plants in the world or at the trader.
As a rule of thumb I would say any plant you consider indispensable should have 8 plots or more.