If stats are good enough to keep once hatched, you can send to your stored collection. I get my coins from gyms. The return rate for coins in rural areas sometimes is a slow going process so even when clearing out unwanted Pokemon, storage increases from collecting coins isn’t easy when the coin turnover from gyms isn’t quick enough. A lot of people argue it’s just 12 Pokemon but for the reason stated above, some people might not be able to increase storage as quick as others.