The minimum amount needed to participate in Stacking directly is dynamic and based on both the total supply of STX and the total amount of current participation. It is be reported by Stacks blockchain nodes at path /v2/pox with the min_amount_ustx property and denominated in microstacks (μstx).

The value will never fall below 50,000 STX.

In various use cases, users can lock up their tokens but might not be eligible for reward slots (multipliers of 90K currently). For instance:

• Stacking a fraction of the reward slot threshold would lock up more tokens than required for the maximum number of eligible reward slots. You can lock up 200K STX but would qualify for 2 reward slots that require only 180K tokens. In that case, 20K would be locked without additional reward allocations
• Stacking less than the minimum threshold. You could use the Stacking contract directly and lock up tokens if you commit over 50K. However, only at the minimum threshold, currently 90k, would you be assigned a reward slot. If you locked up tokens for the maximum cycle amount (12), you would have tokens locked for ~6 months without qualifying for rewards.

If you are Stacking near the minimum, you should review these options.