AD Site - do I need 1 Inter-Site Trnasport or I need more?

Dear Reader,

I have 4 AD Sites connected using MPLS as following:

Site1: DC01 8 MB Connection Speed

Site2: DC02 8 MB Connection Speed

Site 3: RODC01 1 MB Connection Speed

Site 4: RODC02 1 MB Connection Speed

All sites are connected to each other (its like a mesh), currently I have only 1 Inter-Site Transport link.

I need to know the recommendation for the following:

1. What is the advantage and disadvantage of having only 1 Inter-Site Trnasport Link?
2. Is it recommended for my current WAN connectivity to have only 1 Inter-Site Trnasport Link?
3. for those 4 sites, How many Inter-Site Transport links I have to made?
4. In Properties of each of those suggested Inter-Site Transport links, which Sites should be in each link?