Erin Stellato shares information on plan cache sizes:
If you have an ad hoc workload, you’ll often hit the max number of entries before you hit the space limit, particularly if you have the optimize for ad hoc workloads server option enabled, which stores the plan stub for an ad hoc query on initial execution, rather than the entire plan (the plan stub consumes less space, so this is an attempt to reduce bloat).
Click through for the numbers and a couple of options you have around plan cache size.