If by "Mac Pro" you're referring to a MP6,1 (small black cylinder form factor), then Apple requires you to buy 256, 512 or a 1TB SSD. They cannot be upgraded in the field by the user.
However, some have tried this as posted earlier. The OWC does provide a 2TB SSD but it's over $2,000.
If you want more than what Apple provides you can attach am external Thunderbolt-2 enclosure with multiple SSDs and strip them for larger capacity and speeds.