Can the Army force you to have more clothing than issued?

Junk beyond the issue items no. However you must be smart on what battles you chose to fight! Anything that is required for you to have is issued. Now, as you go higher in rank things are expected regardless of issues or not.

When do you get your clothing allowance each year? Yes you can have to get extra clothing. When I was going to SLC in January, we had been given what we found out was an old packing list that said 4 x sets of ACUs (either UCP or OCP) - I went and over two months prior to the course bought four sets of ACUs in the OCP style since I was going to need to get it anyway. Then found out they changed it to 2 x sets. It also had like five PT shirts and shorts or something like that.

If you don't have it you go buy it. You're going to have to have more than the Army issued you in basic training.

If the training or event you're going to requires civilian business attire then you get it if you don't have it. Yes.

Another barracks lawyer? The packing list is there for a reason- to successfully make it thru a deployment, mission, operation. Too cheap to but extra- what if there is limited laundry, and your 1 shirt can't be cleaned in time- is it worth an ART 15 for being out of uniform?