We’d need more information to answer your question specifically. The amount of seed needed varies depending upon your objective.
If you broadcast the seed, three to five pounds will be sufficient depending upon the density you prefer.
Unless Echinacea is grown for commercial purposes, the seeds are usually planted along with the seeds of other native wildflowers to achieve an appealing mixed species effect. In that case, these recommendations apply:
“Planting rates average 10 pounds per acre or 4 to 6 ounces per 1,000 square foot for separate species or mixtures that average 400,000 to 500,000 seeds per pound. Smaller seeds should be planted less, and larger seeds a bit more.”
To obtain best germination, Echinacea seeds need at least four weeks of cold exposure. Fall planted seeds meet this requirement during winter conditions. However, many vendors pre-treat the seeds so cold exposure after sale may not be needed for spring planting.