I've been growing in containers since about 1970 and I've never had a clogged drain hole, or a pot that drained slowly enough that it was a problem. All the clay pots I grow in and at least half of the bonsai pots have one one hole. I give a lot of plants and bonsai starts away, so I often buy inexpensive but attractive bowls to use as pots. They require my adding the drain hole, and I usually drill a hole somewhere between 1/8 to 1/2" in size.
This ^^^ particular pot has a very large drain hole, and I typically secure the piece of plastic canvas (the mesh/screen) with a piece of aluminum wire as seen in the image. The square cut-out in the screen is to accommodate a drainage wick. Even with the very fast-draining and highly aerated media I use, I still like to use a drainage wick like so:
The knot keeps the wick from being pulled out of the pot if I slide it across the grow benches.
Some very small clay pots have holes that are 3/16-1/4" such that no screen is required. I simply tie an overhand knot in the mop strand/wick, then pull the tag end through the hole until the knot/stopper covers the hole. Even then, with the entire hole covered by the knot in the wick, there are no drainage issues.
One thing I would mention is, if you have a plastic pot w/o holes in it, make sure the holes are through the bottom at the sidewall, or through the sidewall so the bottom of the hole is tangent to (touching) the bottom of the pot. Holes only through the side of the pot but above the bottom trap water at the bottom of the pot, which creates issues on several fronts.
Speaking strictly from the standpoint of drainage. It doesn't matter if there is 1 hole in the bottom or twenty. Assuming the pots are the same size/shape and both are filled with the same grow medium, when the pots stop draining there will be exactly as much water in both pots, so multiple holes to make a pot drain faster offers no benefit.
Finally, if a medium supports excess (perched) water, the maximum ht of the perched water table will be the same, no matter what the size or shape of the pot, and changing the number of holes in the container has no impact on how much excess water a planting holds. All pots in the image below are filled with the same medium, and the shaded area in these pots represents excess water.
Al
