In the CryptDB SOSP 2011 paper, they analyzed a trace of queries hitting sql.mit.edu for ten days. I've reproduced their figure 7 here:
Databases | Tables | Columns | |
---|---|---|---|
Complete schema | 8,548 | 177,154 | 1,244,216 |
Used in queries | 1,193 | 18,162 | 128,840 |
These numbers might be useful in estimating the number of tables in a RAMCloud cluster.