When you run a reverse ASIN search in Keyword Scout, by default, the Keyword Scout will include results for all the variants (if there are any on the listing).
If you want to change that, under the 'More Filters' section, enable the 'Exclude Variants' option. Your results will exclude keywords from all child ASINs, so you will only get results for the ASIN you used to run a search.
You can move the slider to the on position to activate (green) or deactivate (grey) this option. If you choose not to enable this feature, your results will include keywords for all variants.
The limit of variants is 500 for the search. So, if you entered 1 ASIN, the Keyword Scout will include up to 500 variants. However, if you queried 10 ASINs, the Keyword Scout will include up to 50 variants per ASIN (regardless of how many variants are in the variant group).