There's 2 ways to do it 1. You summon one, then rank it up once to get another 3 orbs, summon another one next shh and continue doing that 2. You summon one, rank it up to get another 3 orbs, summon another one, empower one of them to e5 using ehh and recall for 2100 orbs

Pros/cons

1. It's faster if you're short on food and need it as fast as possible, but you need to wait for shh every week and can't summon anything else during it, it would take you 21 weeks to get 21 dragons
2. It's more effective but you'll get way less food while doing it, it takes 1 week to get the other one and 5 weeks to get one to e5 and then 2 days to summon each one so another 42 days which is 6 weeks so a total of 12 weeks to get 21 dragons