It looks like it will store as many xattrs as there is space for, based on their contents.
All the file systems that support extended file attributes have some limit for the size of the attributes attached to a file, for instance of 64 kB.
After searching through the patch, it seems that the maximum number of xattrs is 128. There is also a size limit of 128 kB, which might be the limit for the sum of the sizes of all attributes.
If that is the only reason for saying "a limited number", then this is very good, the limits are high enough to not cause any problems in most applications, so tmpfs will finally no longer strip the attributes of the files copied through it.