Hi Lalit
Answer to your first question.
Can we distribute these 8 lines between two or 3 Cisco voice gateways or should all be terminated on same Voice gateway as the D channel is common...?
Since the D channel is common for all 8 PRI lines, all the E1/T1s have to be terminated on the same voice gateway.
1st E1/T1 to be configured with nfas_d Primary since this E1/T1 carries the Signaling with Nfas group 0 and Rlm group 0.
PRI Trunk Configuration
Step 1 
Configure framing, clock source, DS0 group, and so forth.
AS5400(config)# controller T1 3
AS5400(config-controller)# framing esf
AS5400(config-controller)# clock source line secondary 3
AS5400(config-controller)# linecode b8zs
AS5400(config-controller)# pri-group timeslots 1-24
AS5400(config-controller)# !
Step 2 Configure the trunk group label.
AS5400(config)# interface Serial3:23
AS5400(config-if)# no ip address
AS5400(config-if)# trunk-group PRI-TRUNK-GROUP
Step 3 Configure timeslots and preference.
AS5400(config)# controller T1 3
AS5400(config-controller)# framing esf
AS5400(config-controller)# clock source line secondary 3
AS5400(config-controller)# linecode b8zs
AS5400(config-controller)# pri-group timeslots 1-24
AS5400(config-controller)# trunk-group PRI-TRUNK-GROUP-1 timeslots 1-22
Answer to your second question : Pl club all the T1s under common Trunk-group name in each T1 controller, as mentioned in the PRI configuration example and create a common dial peer with the trunk-group mentioned on the T1
Finally answer to your third question : How do we configure dial-peers in such case (H323 gateway).
Create a common dialpeer, to route the calls towards the POTs (8 PRI). Do mention the same trunk-group name
"PRI-TRUNK-GROUP-1" on all the 8 PRI Controller T1s terminated.
dial-peer voice 2 pots
trunkgroup PRI-TRUNK-GROUP-1
destination-pattern 7299......
tone ringback alert-no-PI
translation-profile outgoing typeinternational
no digit-strip
direct-inward-dial
forward-digits all
Thanks,
Lavanya
