Each provider will have different rules on the maximum amount of currency you can buy.
Some providers will allow you to order large amounts, but we suggest you check the terms and conditions before you place an order if you are ordering a large amount.
Also, consider that currently you are only allowed to carry 10,000 euros. If you take more than this you have a legal obligation to inform customs. Note that this amount is per person, so if you are travelling with more cash, you can distribute it to your fellow travellers.