Minecraft wiki entry for effect, associated text: Specifies the number of additional levels to add to the effect. If not specified, defaults to 0. Note that the first tier of a status effect (e.g. Regeneration I) is 0, so the second tier, for example Regeneration II, would be specified by an amplifier level of 1.
Must be a 32-bit integer number. And it must be between 0 and 255 (inclusive).