There is no minimum amount of debt required to file bankruptcy. Legally, you could file bankruptcy on $1; but who would do that? Bankruptcy is designed to provide relief to debtors facing unmanageable amounts of debt. The biggest issue with debt limits in bankruptcy is not the minimum; but the maximum amount of debt you are allowed to file bankruptcy on.
According to bankruptcy law, a debtor filing for Chapter 13 bankruptcy may not have more than $336,900 in unsecured debt or more than $1,010,650 in secured debt. If a debtor owes more than those debt limits, they are not eligible to file for Chapter 13 bankruptcy. Individual debtors who owe more than the prescribed debt limits may be able to file for Chapter 11 bankruptcy which is usually used by businesses.
If you're a debtor considering bankruptcy, these limits are not the only numbers you should consider. As pointed out earlier, if you only owe $1, it doesn't make any sense to file for bankruptcy. But the same conclusion might be applied to $20,000 depending on your financial circumstances. Two people might both owe $20,000, but for one person the debt may be manageable while the debt may be crushing for the other person. Speak to a bankruptcy attorney to find out if bankruptcy is right for you and/or which type of bankruptcy suits your circumstances.