Two should be enough in my opinion. One of storing and one for recovering in case something happens to the first device.
Yes, if I had such an amount I would hold it on different resources partly, of course it is better to use for this the separate device that is not used for different things like Internet or social networks. It is better not to keep the information about your keys or something like that on your gadgets at all and be extremelly attentive while trading. The security of your gadget should be strong enough and it is better to pass through the authentification.
I agree to not keep big amount on just one account, it is more advisable to use multiple account and do not used the same account with trading account because that is very cautious. Use strong security on the gadget and also keep the keys where you can easily access and make it have a multiple copy because forgetting the keys wont have any recovery.