table specific data in django models
In SQL database it would be easiest to create an additional table which would hold a reference to (for example) thread table and user table. Call it (for example) ThreadVisitors.
Whenever a user visits a thread you create an entry in that table for that user and the thread (you could add a unique constraint on (thread, user) pair). That way getting all visitors for a given thread is as simple as running count query (for a given thread). Some indexes would be helpful here and if performance is an issue then you should cache count queries.
You will need such table per each model probably.
Where to store table specific data in django models ?
Here is the scenario
I am making a forum. In each thread, there will be following two kinds of visitors. 

Total Visitors
Current Visitors

My model is designed in the following manner

Category model(will contain sub category)
Sub-Category model(will contain sub-category, foreign key points to category)
Thread model(will contain individual threads, foreign key points to sub-category)
Post Model(will contain individual posts/messages, foreign key points to Thread)

Now, I will have visitors at every level. A user visiting different threads/sub-categories/categories. I want to capture the no. of visitors visiting.
Can anyone suggest me where these kinds of data fit in django model ?