In the third season finale of The Venture Bros, all of Hank and Dean's backup bodies were killed in an epic battle. During the fourth season he says that Hank and Dean no longer have any backup clones left, so they need to be more careful. Why didn't Venture just grow more clones?
The real, out of world answer, I think is they wanted to end that gag. They had been making clones and killing them for years, and eventually it starts to impact how close you are the the characters.
The "in world" answer I guess would be that Dr. Venture felt that having clones was making him sloppy, thus not caring if his children were killed, thus requiring more clones. So best to stop all together and be a good father.
This is not to say that they won't just start cloning again for the story asks for a surprising death of one of the Venture kids.
Rusty had no way of doing so, since he had no way to accelerate the ageing of clones. All the clones we've seen were coming from a finite supply that Rusty created soon after the boys were born and aged parallel to them. There are several situations where this is shown:
- In "Hate Floats", the boys receive the IDs from the previous clones and are celebrating their 16th birthday, but Hank finds it unusual that the year of birth in his ID implies a different age (Rusty immediately shuts him up). I recall it was implied on several occasions they should be around 19 when they are biologically around 16. Presumably, Rusty created the clones around the time the boys were 3 years old.
- In "Blood of the Father, Heart of Steel", Gary (Henchman #24) asks Rusty to clone the recently deceased Henchman #21. Rusty, apart from being unwilling to do so and asking for a large amount of money, points out that the clone would be a baby and would have none of #21's memories.
- In "Perchance to Dean", we see a flashback where Rusty shows Brock the clones as babies soon after he created them.