You can hold and deposit as much money as you'd like on the account, AFAIK there aren't any limits. If you transfer hundreds of thousands of euros in one transaction I'm sure they will reach out to verify the source of the funds, like all serious banks would (to make sure you're involved in money laundering, etc).
It's free to send and receive SEPA transfers. For anything outside the SEPA area you'll pay the fees charged by the third party provider, such as TransferWise (N26 doesn't support non-SEPA transfers directly, you'll have to use a third party).