Your question is a bit confusing - are you asking how far apart to space the pots? Or how far apart in the ground? If the latter, it depends on the variety of rhododendron how far they need to be apart, as some get enormous and some are small.

thank you

Welcome to GOY. Gallon size pots seem to be a bit on the small side to plant anything in. I have azaleas in probably 20 gallon pots.They were hardly more than seedlings and I could have planted more in the pot but as they grow they will not like being disturbed so I planted small annuals beside them (begonia sempervirens). They have been in the pots for about 10 years and more than fill them. I can still squeeze things like Swan River daisies in to the pot to continue the colour through the year. Hope this helps