This is a question which at first glance may seem rather tricky, or even impossible, to answer clearly. However, it is possible to produce some guidance to help with this conundrum. You see, it is not just about the number of plants, it is about the level of genetic diversity to give a good stable genetic population. The picture shows a meadow early in the year with Green Winged Orchids, Anacamptis morio, and dandelions which is a stable population of plants. Some species, like these and Bee Orchids, Ophrys apifera, can have population numbers that vary wildly from year to year, but have long-term stability. These very rarely have what one could sensibly describe as high densities of plants, unlike Common Spotted Orchids, Dactylorhiza fuchsii, which can form quite spectacular groups of flowers. So if you want to set up an orchid meadow, the answer is to put in more of the high density species than the low density ones. By starting off with enough plants in one area of your meadow to give a feeling for what the ultimate goal is, you can be sure that the established plants will seed and spread into the areas which are suitable for them to grow.
There will always be some loss of plants, so setting up a colony in a meadow will need more plants than you might think. Once established, though, your orchid meadow will be self sustaining, the orchids seeding themselves. In the meantime, planting new orchids occasionally will ensure an annual display of beautiful flowers.