For most countries, there is a waiting list of families who want to adopt, so one way to reduce that pool is restricting the number of families who qualify. That could be because of a shortage of available children, or it could be because the country will not allow an unlimited number of children to be adopted for many reasons.
Number of existing children is just one way of restricting families. Some countries restrict by religion, marital status, sexual orientation, income, length of marriage, age, weight, number of divorces, criminal record, etc.
We adopted from China. China has population control policies which limit family size. Their restrictions for IA are the same as their restrictions for domestic adoption. Other countries feel that children who have already been through a lot deserve to have a lot of parental attention and not have to compete for it with a large number of siblings. There are some people who already have very large families (10+ children) who seek to add even more. Some countries are OK with it, and many are not.
Whatever I might think about those restrictions, I do believe that each country is entirely within their rights to determine who gets to parent their kids.
Travel is not required for Guatemala. You may have your child escorted to the US. If you are willing to adopt an older or special needs child, some countries may be more flexible about the number of existing children in your household.