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I think the 68 Gbps backplane supports the 48g port variants plus two 10g ports; i.e. 48+10+10=68.
What's a bit confusing, fabric specifications are normally "doubled" to allow for duplex ports but Cisco, in the posted spec uses the term "backplane" rather than "fabric".
Also confusing, when stacked, there's a pair of 16g stack ports too. It's unclear whether the same backplane or fabric bandwidth is shared with those, or whether there's additional capacity to support them too. I.e. the 3750-E might support wire rate bandwidth when used standalone but might not when stacked.
In any case, when 3750-E units are stacked, the stack ring can be a potential bottleneck. Fortunately, besides StackWisePlus offering twice the stackport bandwidth of StackWise, it also supports (for unicast) destination stripping and also (for unicast) only puts traffic on the stack ring when it must. Still, again, it's a potential bottleneck. It's also a ring, not a fabric.
PS:
Besides bandwidth, you need to check a switch's PPS capacity. I believe the 3750-E has sufficient PPS it can generally support wire rate.
However, one common problem with 2960/3560/3750 series switches is lack of RAM for deep buffering. The 3750 series, I understand, to have 2 MB per 24 copper ports and 2 MB for the uplink ports.
The 3750 stacks are often fine for user edge ports, often not so fine for more demanding roles.
PPS:
Also believe the 3750-E is end-of-life and end-of-sale.
