In MongoDB, how many collections are to many? I have 600 categories i want to track items in, do I create 600 collections or 1, where every item has the field category?
MongoDB does not have many hard limits on the amount of collections you can have, and it depends on what kind of project you are working on. For your usecase, it may be best to have one collection with a field called category as you considered.
Can you tell me the pros and cons of making 600 collections versus making 1 where every item has the field category?   If you can, please provide data on performance using one methodology over the other.   I'd like to know how fast this is going to be.
Having one collection with a category field would be more efficient in terms of performance compared to having 600 separate collections. Here are some pros and cons of both approaches:

Pros of having one collection with a category field:

Indexing: It is easier to index the category field in a single collection, which will improve the performance of queries based on the category field.

Scalability: Having one collection makes it easier to scale your data as your application grows, since you don't have to worry about managing multiple collections.

Cons of having one collection with a category field:

Complex queries: If you need to perform complex queries that involve multiple categories, it may be more difficult to do so with one collection.
Pros of having 600 collections:

Simplicity: Having separate collections for each category may make it easier to understand the data and to perform basic queries.
Cons of having 600 collections:

Performance: Having 600 collections can lead to poor performance since each collection will have its own set of indexes and memory usage, leading to slow query performance.

Scalability: Scaling the data in a system with 600 collections can become difficult and time-consuming, as you would have to manage the scaling for each collection separately.

In conclusion, having one collection with a category field is likely to be more efficient in terms of performance, as well as easier to scale and manage. If you need to perform complex queries, you can still do so by combining multiple indexes on the collection.