Planning and scheduling your sowings is important not only for your greenhouse space but for your amount of time that you need to spend with them and for your future crops.
First of all you must work out exactly how many plants you think you will need in your garden. According to this number you will know how many seeds you will need to sow. A general rule is to sow twice as many seeds as you need plants. This will accommodate any losses and allow for a 70 per cent germination rate, which is the minimum standard for commercially produced vegetable and flower seeds.
Most of seed packets, especially the ones with small seeds, contain hundreds or even thousands of seeds, so a single packet will contain enough seeds for several seasons.
The problem that may appear in this case is that once the foil pouch inside the seed packet is open, the viability of the seed germination deteriorates with time. If the remained seeds are stored well, you could expect a 50 per cent germination in the second year and about 30 per cent germination in the third year. Always seal back the packet right after you took out the necessary amount of seeds for your sowing and place it in an airtight container and keep it somewhere cool.
To avoid this problem you can buy your seeds together with some friends and share the seeds from a packet and buy fresh seeds each season.
Do not sow all your seeds at once because all the seedling will then need pricking out and potting on at the same time and will require more space to grow at the same time. You should divide your sowings in small batches over a few weeks. This will give you more flexibility. For an even more organized activity you can make yourself a seed organizer, divided up into weeks throughout the spring and summer. Slot each packet of seeds into the week you plan to start sowing it, then move it on to the next sowing date after you have made the first sowing of that particular type.
If you end up with too many seedlings, give them away rather than growing them on yourself, else you will end up with extra cost for pots and compost and valuable space loss to grow them on. Pick only the strongest seedlings for pricking out because they will perform better in the long run.