I want to buy with company to get big quantity of crypto
And do i need to declare taxes of this ?
By means, which country you're staying in? that would be needing you to research about taxation but AFAIK, you'll be taxed after you get a profit when you sell.
Why you have to make a company if being an individual will allow you to buy from different exchanges or OTC markets? Do you just want to be labeled as an institution that purchased bitcoin? does that matter to you?