That said, two practical considerations are worth keeping in mind.
Your investment needs to be fundable
Alternative assets have their own minimums. A private placement may require a $25,000 minimum investment. A real estate purchase requires enough capital to cover the purchase price, or at least the down payment if using a non-recourse loan, plus carrying costs and reserves. Tax liens, cryptocurrency and private notes can be acquired with smaller amounts.
Before funding your plan, have a general sense of what you intend to invest in. Your target asset class should drive how much you fund at the outset, not the other way around.
Your bank may have a minimum balance requirement
Your checkbook IRA or Solo 401(k) will maintain a checking account in the name of your LLC or trust. Some banks require a minimum balance to avoid monthly fees or to open a business account. Requirements vary; many business checking accounts have no minimum at all. This is worth confirming when you select your banking institution.
You can start small and add funds later
If you're not ready to fully fund your plan from the start, that's fine. You can fund minimally at first and rollover or transfer additional funds once you have a specific investment under contract.
A common example: you know you want to purchase a rental property, but you haven't identified the right one yet. Fund your plan with enough to cover an earnest money deposit and pre-closing costs like inspections, then take your time finding the right deal. Once your plan is under contract, transfer the remaining funds needed to close. This keeps your capital at work in your existing accounts while your plan stays ready to act.