You are correct that an MME can be configured with max 32 PLMNs and eNB can be configured with max 6 PLMNs. Both these are operator configurable. Now, before broadcasting the PLMNs, eNB cross-checks if it's configured PLMNs are matching with the PLMNs in the MME's list. It broadcasts only those PLMNs which pass this check.
I believe, an MME can have a very large tracking area and within that different eNBs can be configured with different PLMNs by operator. That may be a reason why 3GPP has made the standardization in this manner and have chosen the numbers.