sc-executor: Increase maximum instance count (#1856)

Changes the maximum instances count for `wasmtime` to `64`. It also
allows to only pass in maximum `32` for `--max-runtime-instances` as
`256` was way too big. With `64` instances in total and `32` that can be
configured in maximum, there should be enough space to accommodate for
extra instances that are may required to be allocated adhoc.

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Co-authored-by: Koute <[email protected]>