The current example for a count query at https://redash.io/help/data-sources/querying/mongodb/ doesn't seem to work, as of v25.1.0 the count query also requires a query object. I'd propose the example is changed from
{
"collection": "my_collection",
"count": true
}
to
{
"collection": "my_collection",
"query": {},
"count": true
}
to reflect a minimal working version of a count query.