There is no minimal annual contribution for the Solo 401k. As an owner-only 401k plan, the business owner (you!) determines the amount to contribute each year based on net income.
You do not need to contribute each year in order to keep the plan active. However, your sponsoring business (that you own) must remain a legitimate business with an intent to profit.
It is generally recommended that you have some type of business activity each year to keep your Solo 401k active; however, even if you're earning income with your business, contributions are not required.
For Solo 401k plans provided by Solo401k.com, you are 100% vested in your plan once you start working for your business, so you can make contributions each year, or skip years in where you're not earning enough for you to want to make contributions.