I have a google spreadsheet where I keep info on all of my plants. For each plant I record the specific garden it is in and additional details on its exact location (e.g. Northwest tip of garden G4). If I am planting many of the same type of plant in the same garden, I sometime identify a plant in the middle and then specify the others in relation to the middle. For example NW, N, S and so on from the middle. This method works well for groups of 9 or less. Of course, if I plant them in a line, I can number the plants. For example, in my V1 garden, I have 13 daylilies planted in one row so I numbered them.
For plant markers, I use 4-foot fiberglass electric fence posts from Tractor Supply, which I cut into 9.5-inch rods, tapering one end for easy pushing. These 4 foot rods are only $1.29 each. I make the labels using a Brother P-touch label printer and attach the laminated tape label to one end of the rod.
These markers last a long time and they are unbreakable (you can use a hammer to "push" them in).
Another advantage is that you can make the marker as long or as short as needed. For example, I use the entire 4-foot pole for my dahlias. As an added benefit, a 4 foot marker can double as a plant stake.