Single DHCP server using Airport Express as primary router and Time Time Capsule in bridge mode I use an Airport Express as primary router - i.e. attached to the modem - and a Time Capsule for back up but also as secondary router in bridge mode to extend the wireless network beyond the reach of the Airport Express.

I am planning to add a NAS to my LAN (via Ethernet to either of the two routers). This requires that I only have one DHCP server - i.e. that the two routers are seen as a single subnet.

Does the configuration above with the Time Capsule in bridge mode result in two DHCP servers or only one (the primary router - Airport Express - whilst the Time Capsule does not act as a DHCP server)?

Thanks.
Your setup as described contains one DHCP server on the Airport Express. The Time Capsule in bridge mode basically acts as a repeater and does not assign IP addresses or add a separate subnet.