It's just the ones in Britain, I understood.

they are considered national possessions, Harry, weapons in case of war.

Ah. I made an assumption here, but from this I got that they kept their Dementors in reserve so that they would not lose an advantage that their enemies had. But an equally applicable interpretation would be that they did not want to lose an advantage that they had over their enemies.

Keeping that in mind, however, I would rather doubt that other governments would allow Britain to have such an exclusive advantage, not when the weapons are all held out in the middle of the ocean. Though that assumes that all of the other governments don't have their own exclusive weapons...