It's a toss up really, and if your lucky, you will get 18 with each round! You can do the math for that but its so unlikely to ever achieve such luck! I say 30-40 Gants per Tervigon is a safe bet. Most of the time you can replenish your "reserves bank" with the death of previously spawned Gants. I believe the average is ~9 per round, so 9 x 6 rounds equates to 54 but that really is overkill. The more Tervigons you take, the more Gants you will need.
Should sum it up. For friendly games, 30-40 should fare you well. In tournaments, you need to decide if its better to be safe or sorry and go for the max potential and have them available.