Author here,

Nim supports variadic generic, it's an arbitrary limitation so that shape and stride small vectors that describe a tebsor can be stack-allocated and fit in a cacheline.

Also at the time, Nim default heap allocator was not compatible with OpenMP.

Edit: it can be configured via a compile-time flag to 8 or 10 or anything: https://github.com/mratsim/Arraymancer/blob/master/src%2Farr...