Naturally this depends on the kind of display you wish for your garden. Do you like little dots here and there, or do you prefer large drifts of flowers? If you plant bulbs in drifts the distance can be one bulb width. If you like a loose weave, the distance should be 3x bulb width. Bulbs with large flowers, like Frittilaria Early Dream, can be planted 20-25 cms apart from each other. In the gaps you might want to plant some smaller bulbs like grape hyacinths or crocuses. Liatris Spicata should also be planted 20-25 cm apart.
For crocuses we advise between 20 en 200 bulbs per square metre, depending on the effect you wish. In a lawn 20 bulbs will be sufficient, but for the effect of a carpet of flowers, you might need up to 200 bulbs.
For a full floral display of tulips or daffodils you need circa 100 bulbs per square metre.