Bamboo clumps can become very large and dense. I've never had to maintain one from the get-go, but I'd think it's something you need to keep thinned, such as removing emerging culms as they begin Spring growth. as it grows in order for it not to get too thick.

There is no definitive limit to the spread. The size of the clump will continue to expand over time. Clumpers just do not send out long horizontal root shoots like a running bamboo does.

If in a tight space, I would advise keeping it contained, just for ease of maintenance.

How much space do you have? I have a number of clumping bamboo that I planted 15 years ago and they are probably 3'-4' in diameter. I don't have your clumping variety (Bambusa multiplex) because it's not quite hardy enough for my Zone 8 location. However, most clumpers are not going to expand radically. I would opt for pruning -- years in the future -- if your clump get too big. Installing barrier is something I only do for running bamboo; clumping bamboo typically does not need it.

I have Fargesia robusta, from a seedling with 2-3 tiny culms and it's now 3-4 foot wide in 5 years. True to form, last year it really expanded and I think it will do more so this year..faster and faster.

I second the thought of container grown.