There are minor differences depending which variety you've got - Cornus alba 'Elegantissima' for instance gets much taller than, say, Cornus alba 'Sibirica', so knowing the variety would be helpful. As would knowing what you're using it for - just group planting? Hedging?

It also depends greatly on the effect that you are going for. Cornus alba will ultimately form a huge, sprawling shrub, but virtually no-one grows the plant without cutting it back hard every so often to promote the production of the coloured stems.

If that's your plan too, then you can space them really pretty close to one another - around 1 metre - for maximum visual impact.

If you plan to let them reach larger proportions and grow more naturally then spacing of 2 metres will see them join up with one another in around 5 to 7 years, and wider spacings still will allow more room for ongoing future growth.