Planting live plants in anything smaller than a 3 gallon is just irresponsible and stupid. I've seen pictures all the time of fancy home designs in magazines or on Pinterest that have tiny fish tanks all around with one betta or guppie each, and some with a huge plant. All of us hobby aquarium keepers know that that is immoral and no fun for the fish or the hobbyist. Three gallon is the minimum with the only exception being moss balls.
Once we get a little higher than three gallon it starts to open up some doors for more elaborate live plantings. It also allows for some really cool dedicated aquascaping which is really what our eventual goal is. In a five gallon tank consider the placement of the plants and the focal point you want to create. Starting at that size, a lot of aquarium owners love to make a piece of driftwood or lava rock the center piece with the plants surrounding it but not overbearing it.
When you get to a ten gallon or more tank you are able to take advantage of beautiful lush landscapes of live plants like you never imagined. Though you will have to keep in mind, limiting natural light because it will cause algae outbreaks, and the use of C02 since you will have so many plants in there working so hard to thrive.
In the bigger tanks I've always enjoyed creating a full background. Instead of buying those neon colored backgrounds with photographs of someone else's tank, why not fill your own? Look for tall and full plants for the back. In the mid ground leave pockets of space open so that you can view your fish and create more natural focal points. I love placing smaller river stones in those locations, giving my snails a place to hide. And finally in the foreground place the smaller mosses and java ferns that are short and stubby. You don't want to hide your beautiful designs with plants that grow way to tall and bushy.
So what's the rule? There really is no rule. Just consider the space your fishes need to swim in to be comfortable and enjoyable for both you and them. Stuffing a bunch of live plants in a one gallon or less tank is really not a good idea. Three gallon is your minimum for planting and should be limited to just a couple of breath-taking or forgettable plants, depending on what your centerpiece is. After you get past five gallons the sky is the limit. Just remember it's more maintenance for you then.