Session State is a critical component of the Sitecore ecosystem.
Redis is typically leveraged in this regard, alongside the correct Redis settings, and the scale of a C1 only offers “low network bandwidth” which can create bottlenecks, especially when under Production load.
While the total Redis cache size may appear adequate, it is this oft-overlooked pipeline issue that has caused multiple Production issues. It is therefore recommended to scale to at least a C2 which provides “moderate network bandwidth”.
If you are unsure what size of Redis you may need, conduct load testing prior to go live… or load test in a Staging environment of similar size with the same networking components and code to determine your Redis needs.
Additionally, as Sitecore Commerce builds tend to have two Redis instances, one for XP and one for XC, so make sure both are properly scaled to avoid bottlenecks.
A breakout of the Redis sizes is as follows: Pricing – Azure Cache for Redis | Microsoft Azure
For more options to tune your Redis, check out the 4 Quick Redis Tips to Optimize Your Sitecore Environment article.