Why are funds reserved?

To prevent spamming and to ensure that the blockchain is not bloated with useless data, some funds are reserved as soon as some data is stored on the blockchain. For example, you will get reserved funds if you set an on-chain identity, do a multisig transaction or create a pure proxy.

Multisig transaction

When you create a multisig transaction, the transaction data is stored on-chain until it's approved or cancelled. On Polkadot, the reserved funds are ~20 DOTs. The funds are returned when the transaction is approved (with enough signatories) or cancelled.

Creating a Pure Proxy

When you create a pure proxy, the chain must store the association between the pure proxy and the account. On Polkadot, the funds reserved for this storage are ~20 DOTs. The funds are returned when the proxy is killed.

Summary

If you are creating a multisig with a proxy, you will need to have the funds to pay for the reserve of

• the transaction to create a pure proxy (returned once it's approved)
• the pure proxy (returned only when the pure proxy is killed, which may be far in the future)

For this reason you will need ~40 DOTs, 20 of which will be returned in the short term and 20 of which will remain reserved for as long as you need the multisig/pure.