there is a lock option in mysql ..
but it's not what you want
http://dev.mysql.com/doc/mysql/en/lock-tables.html
I don't think MySQL offers the feature you are asking for..
but you could do that with the help of php ..
you could count the number of rows in each table, with MySQL's function COUNT() and if it reached the limit you specify, then start in a new table ..
but why would you do that anyway? this will cause you trouble retraiving data as you are going to search in all tables!
database servers came to overcome this problem which was faced when using huge flat files .. but with databases, if you have a good structure, your table could handle millions of records!