1. In a enterprise, multiple area ospf network environment, why do I see that there are multiple DR and BDR.
172.16.97.2 1 FULL/BDR 00:00:34 172.16.255.30 GigabitEthernet6/8
172.16.97.1 1 FULL/BDR 00:00:35 172.16.255.26 GigabitEthernet6/7
172.19.1.2 1 FULL/DR 00:00:31 172.16.255.38 GigabitEthernet6/10
172.19.1.1 1 FULL/DR 00:00:34 172.16.255.34 GigabitEthernet6/9
2. what is the pro and con of such design
3. how many DR and BDR that a area can have?
Your question seems to want to treat DR/BDR as related to the OSPF area. But the Dr/BDR is related to subnets/LAN segments. There is a DR and perhaps a BDR on each multiaccess segment (typically an Ethernet/FastEthernet segment or some other type of multiaccess segment.
So the technically correct answer to your question is that an area can have as many DRs as it has multiaccess segments in the area.
As rick says the DR and BDR is elected on per interface basis.Dr/Bdr is elected on multiaccess network type, so in an area the number of DR/BDRs depends on the number of multiacess interfaces you have in that particular area.
So if your router has 2 segments in different/same multiacess networks it can be a Dr/BDR on 1 segment at the same time it can be a DRother router on another multiacess network.Per segment an OSPF process can have a single DR with no BDR or a single DR and a single BDR.
If a Gigabitethernet interface is connected to the neighbor router or layer3 interface on a multilayer switch as a point-to-point connection, it is safe to configure it as ospf point-to-point type interface:
Interface Gigabitethernet 6/10
ip ospf network point-to-point
In this case no DR or BDR is elected on the network segment, as it is not necessary. Neighbor relationship will form anyway.
This way you can save some memory and processor cycles for the routers.