This is most likely a problem with your query. Lets take a look!
If your query looks something like this:
tag=shodan json port==502 limit 2
This works in the sense that the query will run, but not limit results to 2.
The above query is missing a pipe, so its really just telling the json module that you also want to extract fields named "limit" and "2" vs actually limiting the number of records returned.
Instead, try adding a pipe:
tag=shodan json port==502 | limit 2
This will return two records.
Here is a link to some documentation about the limit module in Gravwell: