1)
What is "Moshav Beis Tziva"?
1.
Rashi: It is his sons and servants [Radak - and household].
2)
Since Mefivoshes had only one son, why did he need so many slaves?
1.
Malbim: Many friends and acquaintances had gathered to him, like is common for nobles who sit foremost in the kingdom. Much work was needed for them.