Q. How many virtual networks can be peered in Azure?
Dept - Azure
A. Virtual Network peering was GA'd in September 2016 and enables virtual networks in the same region (at GA) (but can be in different subscriptions) to be connected together without having to use site-to-site VPNs or ExpressRoute. One of the virtual networks must be an ARM virtual network and the performance between virtual networks is the same as for VMs within a virtual network. Remember that virtual networks that are peered are not transitive which means VNET1 peered to VNET2 and VNET2 is peered to VNET3 does NOT mean VNET1 can directly communicate to VNET3. A separate peer is required between VNET1 and VNET3. Note that if this type of behavior is required then transit routing can be used with a virtual appliance running in VNET2 but this is not native to Azure and would required the virtual network to be enabled to allow forwarded traffic.
Each virtual network can be peered to up to 10 virtual networks.
There is no gateway required but there is a cost for virtual network peering based on the ingress and egress data. This is documented at https://azure.microsoft.com/en-us/pricing/details/virtual-network/.