Sadly, we don't often see collards going to seed in our climate. It seems to be a rare occasion and by the time they do, they are infested with the emergence of spring time aphids. Collards are a biennial crop, which means they need two seasons of growth and a true dormant winter period in order to trigger seed development.
However, in our subtropical climate, collards will produce seeds during winters that are unusually cold, with extended periods in the 50F range. Some winters we have that, and collards can be successfully grown for seeds, and that's how we are slowly building our Florida-grown seed supply. Very mild winters with little extended cold, will be less likely to support collard flowering and seeding.
Brassicas within the same species can cross-pollinate if they are flowering at the same time. Brassica oleracea is a diverse group of vegetables to which collards belong, and if you are growing more than one type and they flower at the same time, they will most certainly cross (for example with broccoli!). This is only a concern if you are attempting to preserve a specific variety.
For long-term seed saving, we recommend having at least 20 plants flowering when you collect seed. More is even better! When growing specifically for seed, we often grow 80–200+ plants to maintain a broad and healthy gene pool.
If growing that many plants isn't practical for your garden, don't worry! You can simply let a few plants flower and provide a valuable food source for bees and other pollinators, then cut them back once they're finished flowering.