What is the maximum bandwidth utilization (in percentage) in order not to reach congestion? Starting from which...
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.