That is, simply storing 51% of the total amount of Bitcoins on the wallet will not help to make an attack?
The number of coins you own is irrelevant to a 51% attack. You can attack the network despite not owning any bitcoin.
Do you need exactly 51% of the capacity of bitcoin, computing machines that ensure the operation and security of the network?
No. You need a sustained minimum of 51% of the total hashrate to guarantee that your attack will be successful over a long enough time frame. You can have much more than this, but you could also try to attack the network with much less than this, although your chances of success exponentially decrease.