Usually how low of t-cells befor wasting occurs.

Research suggests that wasting occurs for many reasons and independently of T cell counts. Simplified: wasting happens in people with high T cell counts and low T cell counts and is more related to things like infections and eating habits. If you have low enough T cell counts and tend to get infections that are common with low T cell counts, wasting can be round-aboutly related.

If your T cells are low and you end up with opportunistic infections you are at a very high risk of wasting. The wasting that happens then is often a combination of problems: your body needs to break down some of its protein stores (muscle being the first target) to activate the immune system and the process to do this can cause a loss of appetite (those little immune activating proteins called cytokines play a role). So you end up breaking down muscle and not eating enough to blunt that response. While this process needs to occur for your body to protect itself, you may be able to blunt the severity of wasting by making sure that you eat enough calories and protein even when you don't feel like it.

It is a bit more complex that what I have written here, but the point is still the same: eat well regardless of infection and you will tend to do better, recover more quickly from illness, and keep wasting to a minimum.