The theoretical limit on the number of resources an application can open at one time greatly exceeds the practical limits of a program.
If you are not using "owned" resources, such as resources for desk accessories, you can have up to 32767 of a single resource type. If each of those resources held 1Kb of data, you would be accessing on the order of 30MB. In other words, the limit on the number of resources you can have open at one time is not a real limit in the way that the limit on the number of open files is.
If you are considering having thousands of resources open at one time, we suspect that you need to take a hard look at the way you are doing things and develop an alternative method for accessing data.
Note: Tech Note #141 states that the maximum number of resources you can have in a single file is 2727. Also, be aware that the size of a resource fork of a single file is limited to 16MB. This probably isn't an issue, but it has been brought to our attention at least once.
The following Tech Info Library article can help you find the Tech Note mentioned here:
Article 24493: "
Apple Tech Notes: What They Are, Where To Find Them"