I'd go for 3 in a 10" pot as they will bush out. they don't like to be wet all the time but certainly not dry compost. they need good drainage really so if you are using multipurpose compost I'd add a little vermiculite or grit to improve drainage. then when outside and it rains they wont sit in very wet compost. dead head spent flowers regularly to keep them forming flowers. if you have slow release fertilizer granules add a teaspoon[ish] of them and mix into the compost before potting up your pots.

Thanks SBG that's great advice for me and I know it's reliable. I'm going to order some slow-release fertilizer granules because I see them regularly being advised on GOY. I think I have some vermiculite so i will incorporate that too. Thankyou very much.

Have just looked on Amazon for slow release fertilizer granules. They say they last for six months. Does that mean one doesn't need to use high potassium feed during that time?

yes I just use the slow release stuff then in the autumn I check the pots for vine weevil and in spring add more granules if the plants are still alive.

Yes, three in a 10 inch pot is just right and the slow release granules will do the job. I grew this new variety last year and was surprised that they went through the winter. They will cope with most composts and even if they get dry they will survive. By the way, they will, like most dianthus, come easily from cuttings.

Thanks Jimmy. I'll take some cuttings just in case.