There are usually some that never come up. a few genuses have as little as 80% germination rate. However that is not Tomatoes or leeks. I always plant a few more seeds than the number of plants I want.

Usually I give up when I'm ready to transplant the rest into bigger containers. The ones remaining may be duds or if they do sprout be weak plants. To hold off waiting for 1 or 2 that may never show would be detrimental to those that are already viable plants.

If you do transplant your tomatoes remove the seed leaves and sink them into the soil up to the first true leaves. They will grow roots all along the stem. This is not true of most other plants but with Tomatoes it is. Don't sink them when you set them out however. You don't want the roots down in cold soil.

Thank you so much for your kind advice!!