Well, initially, your concern should be whether you can effectively reach/market to your target niche and whether they buy. If you run out of design-X in size-Y, well good for you! You see it often that a design is out of certain sizes or color options. The world will continue to revolve.
If you actually manage to sell some shirts and seem to have a few designs that are winners, then you might consider buying a cutter and press and doing the HTV yourself. Then you would just cut/press shirts as they were sold. And/or if you can justify ordering around 100 units per design, then you could have your better selling designs screen printed.
If you actually manage to sell some shirts and seem to have a few designs that are winners, then you might consider buying a cutter and press and doing the HTV yourself. Then you would just cut/press shirts as they were sold. And/or if you can justify ordering around 100 units per design, then you could have your better selling designs screen printed.