Read these guidance notes and tips by Rikk Flohr (Software Quality Engineer for Lightroom ecosystem of apps, Adobe).

HDR photos are used to capture scenes having a large dynamic range. However, using more number of photos can lead to unwanted artifacts from poor alignment or ghosting. For optimal HDR merge, the aim is to capture photos in a manner that each part of the scene is well-exposed, that is neither blown-out nor under-exposed in at least one of the photos.

Use the following guidelines to identify how many photos work best for your case:

• If your HDR bracketing is less than 3.0 stops in total separation (-1.5, 0, +1.5), use only the darkest and brightest exposures to generate an HDR. Capturing the middle exposure, or zero exposure, is not necessary for generating a quality exposure blend in such cases. If you exceed the 3-stop separation between the darkest and the brightest exposures, an additional exposure offset becomes necessary to process a good quality HDR photo.

• If you are a photographer using the ± 1.5 exposure bracket, you can ignore the zero or middle exposure. This helps in faster render and improves alignment odds in the resultant HDR by reducing the chances of potential camera movement between the exposures. However, the zero exposure can be useful in scenarios where the capture scene is within the acceptable range of a single exposure and can be developed independently.
• If you are a standard HDR shooter using a ± 2.0 bracket, you ideally require only three photos to merge into an HDR. 
• If you are a 5 shot ± 4.0 stop shooter, you can now drop from 5 shots to 4 shots for merging and processing HDR. However, if you are a 7 shot ± 6.0 stop shooter, you can now get the optimal HDR blend with only 5 shots (-6.0, –3.0, 0, 3.0, 6.0) provided your camera has three-stop stepping in the exposure bracketing function.