The maximum order of the interference pattern that can be found before the pattern is obscured by the first diffraction minimum is ±2.
The maximum order of the interference pattern that can be found before the pattern is obscured by the first diffraction minimum depends on the system described in the question. In the given example, an interference pattern is formed by waves from two slits, and a diffraction pattern is formed by waves within one slit. The maximum order for the interference is missing when it coincides with the minimum of the diffraction.
In Figure 4.11, the maximum of ±3 order for the interference is missing because the minimum of the diffraction occurs in the same direction. Therefore, in this particular case, the maximum order of the interference pattern before the pattern is obscured by the first diffraction minimum is ±2.