Is it better to have a single Direct Connect Gateway or multiple Direct Connect Gateways?

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2 is impossible. It is not possible to attach multiple Direct Connect Gateways to one VPC-VGW. Therefore, 1 is the answer to this question.

A single DCGW is generally better for most use cases.

For connecting multiple VPCs (VPC-A, VPC-B) to multiple data centers (DC-A, DC-B), the recommended architecture is a single AWS Direct Connect Gateway (DXGW) paired with a Transit Gateway. Each data center establishes its own dedicated DX connection — ideally terminating at separate DX locations for maximum resiliency — with transit virtual interfaces pointing to the shared DXGW. The DXGW then associates with a single Transit Gateway, which has both VPCs attached, providing a centralized, scalable hub for hybrid connectivity. This single-DXGW approach simplifies management, reduces operational overhead, and aligns with AWS Well-Architected best practices, while the Transit Gateway enables inter-VPC routing and scales to thousands of VPCs if needed. Multiple DXGW should only be considered when strict routing isolation, regulatory separation, or DXGW association limits require it — none of which apply to a two-VPC, two-DC setup.