Extensive Tempo Workout Limit

What is the accepted mileage limit for an individual tempo session. Would it be okay to execute a Big Circuit and Small circuit in the same tempo session, if you are only doing Tempo once a week?

depends on what you are used to, what event you are training for and how fast you are planning on doing this etc…
Work of any sort is best when spread out over the week.
Are you doing this to save time?
Have you read any of the training materials from this site?
As a back ground to what the top track athletes in the world typically do reading Speed Trap is a quick read with an overview of how to train.
hope this helps.
btw, experienced athletes who require larger amounts of tempo for their event have done 2 sets of big circuit with 5 to 10 minute break.
I’d like to know what kind of time you are running this in for one Big Circuit before understanding how much tempo in one session.

Yes, it is safe to add more tempo to suit your needs. If you need extra work though you can always add some body weight calisthenics and/or push-ups/sit-ups in beteen each rep.

But as Ange said it depends on the event. I find I vary my tempo sessions from 1000 and up to 2000 meters in a single session.

my body feels freshest after doing 1500m of tempo, and this is enough to get me lightly panting

i have done 4000m in a session, but anything more than the big circuit alone has acted for me as condition but not recovery at all, which if speed work is your priority, at least from my experiences, more tempo just because you can do it isnt necessarily better