Depending on what sort of Fruit tree will depend on how far apart they are planted:

Apples, pears and plums are planted 3-3.6m (10-12ft) apart.

Cherries, medlar and most nuts are planted 4.5m (15ft) apart, except walnuts which are spaced 7.6m (25ft) apart.

Apricots and peaches should be planted 2.5m (8ft) apart and if the are trained as a fan they should be 30cm (12") from the wall.

Plant grape vines 90cm (3ft) apart.

Kiwi should be spaced 1.2-1.8m (4-6ft) apart.