What is the maximum bandwidth utilization to avoid congestion?

percentage of used bandwidth do we consider that the bandwidth is not sufficient anymore? A good answer to this question starts with a definition of "utilization," For starts, consult an answer to a similar question asked some time ago: https://searchnetworking.techtarget.com/answer/Measuring-utilization and https://searchnetworking.techtarget.com/answer/Standard-for-bandwidth-utilization-over-WAN-circuit

Finally I would suggest a look at my white paper on traffic just to get another perspective on the nature of congestion and its sources. https://www.bitpipe.com/

All that said, and without getting any clearer about the nature of how you actually measure utilization, some typical numbers (extracted from another reader's question) could be
<38% = stable
38-42% = borderline
42-47% = concern
>47% = definite upgrade

Knowing where most people get those number (MRTG for example), this scale is appropriate for typical networks. That said, this is a very broad rule-of-thumb and your mileage will certainly vary.