Memphre is right these stamps are routinely issued in small numbers, and are given to VIP, including members of the Royal Family. They are normally numbered at the back, and example #1 is reserved for the King. Also, for the most prominent VIPs, they are given in blocks of 4, which is even rarer.
The information I'm giving here pertains to Belgian stamps, but as Memphre mentions, both France and Belgium have a similar issuing policy for imperforates (with the obvious difference that there is no Royal Family in France).
Currently, 1000 examples of each new stamp are produced as imperforate. This wasn't always the case. Before 1971, smaller quantities were produced.
In the late 60's the quantities produced varied from 370 to 480.
Between 1958 and 1965, the quantities varied between 210 and 320, but with much smaller runs for souvenir sheets (100 to 120), and the 1963 booklet (only 70 were produced).
Between 1934 and 1958, the quantities issued were in the 200 to 300 range, and before that, between 50 and 150.
To echo Glen and Memphre, these are great to collect, and while they are generally inexpensive, it is harder to find those before 1958 (and they are of course more expensive).
It's also harder to find the blocks of 4 (much rarer), and examples with plate numbers (very rare of course, as there is only one per sheet, so even with a production run of 1000, there are only 33 available. Much less for smaller production runs).
Some themes are also very popular and hard to procure: the Olympic Games of 1920, with only 100 produced, routinely bring in high prices at auction (virtually impossible to get a set below 500 Euros)..and try getting the unique plate number from this set, since there is only one sheet of 100 produced...(Oh wait, for two of the three stamps, you can't, because they are in my collection...
, ok being a bit cheeky there, and to be fair there are rumors that two other sheets were produced. Harder to verify because they were produced in the US, so less reliable information in Belgium on that...).
For the Tin Hat set of 1919, only 50 sets were produced, so plate numbers are exceptional: only two sets exist, on of which was in the Karpov collection and was sold by Feldman a couple of years ago.