Bulbs come in all sizes, from tiny snowdrop bulbs to the large Alliums.
Planting too shallow can cause bulbs, particularly Daffodils, to come up blind. A good rule of thumb when planting bulbs is always to plant them 3x the depth of the bulb. Looking at the bulbs in the image, the large white ones are Hyacinth and all around are Daffodils.
It is clear from this that Hyacinths will need to be planted deeper than Daffodil bulbs, and the smaller Daffodils bulbs, such as Tete at Tete, not so deep. If you are planting a few, it is handy to measure on a bamboo cane the 3x depth, and then each time you dig a hole put the cane in to check the depth and then plant.