You don’t need any. Anyone can run a node and earn mana if others use it. How it works is, token holders select a node they trust to issue transactions. They pledge their mana to that node. The tokens don’t have to be yours. You can run a node without possessing any tokens and if someone wants to use your node you’ll earn mana. The more mana you have the more transactions you can issue and the more your vote in consensus matters.