Hybrid teas take more space than floribundas. Old garden roses can grow to massive proportions. Rose books will offer a guide to the eventual size of your rose variety. As a general rule plant floribundas at least 24 inches apart and hybrid teas at least 30 inches apart. These are bare minimums.
More space is better.
Organic rose growers argue that crowding roses encourages black spot. Place the pots in the desired locations, using the ruler to check spacing. Dig the holes one at a time, not all at once. This allows minor adjustments in spacing as one proceeds. Use the rese in the pot to determine how deep to make the hole.