There is no such limit.
You are limited by your machine and network latency: when database
tables get too large and it takes more than 1 minute to return a query,
the connection will time out.
You are limited by database connections: MSDE has a limit of 5
concurrent connections running in the database. Later requests get
queued up. You'll notice a slow down if say several users
simultaneously run reports from BCM or do imports.
You are limited by absolute amount of data: MSDE has a limit on
database size (2 GBytes?). How many records you can have before you run
into this limitation depends on your entity ratios (e.g. number of
Activities to Accounts) and how much data is in each record.
These last two aren't strictly BCM limits, but a SQL Server licensing
issue. MSDE is free. If you buy a SQL Server license the issues go
away. With a license the server can also use more CPUs, so issue 1 is
also mitigated.
In my experience, with a typical small business PC's RAM and CPU, with
a typical mix of processes and services running on the machine, the
first limitation is the one you'll run into.