Hi Sultan,
1) Size : 30+ or 300+ what will hit your scalability is the capacity of your PE devices where you are termiating these 30 locations. if its a full mesh the PE have to be of comparable capacity.
Having said that, how many routes you are expecting per site and in total, and what is the VPNV4 route capacity of the devices has to be looked at.
2)Redundancy: You may also want to look at the redundancy part of the VPN's if there are 1 or 2 sites or these 30 as central locations, thye may need redundant connections to different PE's, where you may have to loadbalance the routes.
3)Topology: Whether these 30 sites are full mesh or hub and spoke or they require some central site services, based on that your design needs to be worked out considering the points mentioned in the above 2 points.
ALso you may want to check the considerations as whether they have existing backdoor connections which they plan to use.
To sum up, at first glance its the though of scale, then other requiremnt considereations.
Coming to your possible issues or feasibility, the issues depend on your answer of the first two points, and feasibility, should not be a problem, I hope if I tell you 800+ sites can be deployed in a hierarchial HubnSpoke topology in a stable manner, that should take care of the feasibility concern.
Also if you VPN is Internet VPN going by your subject, then also the same major considerations as specified in the first 2 points hold.
HTH-Cheers,
Swaroop
