Hi, welcome to Go Y, I presume you're refering to bedding dahlia's, as the decorative larger flowered ones are best propagated rom cuttings.
Sow the seed in trays, {if you have the cell type trays, put 2 seeds in each cell, and when they germinate, remove the weaker o the 2} in early spring at 61 deg f, grow them on under glass before hardening them off, and plant out when the risk of frost has passed, Derek.

If you only have pots it depends what size they are. You might sow them about an inch apart and them move the best ones on to separate pots when they are big enough, discardibng any obviously weka ones. Bear in mind theough that the really vigorous ones are probably the more usual colours!