Not sure how many tourists come to the island each year. Not sure if anybody knows the exact number. Tropic Air and Maya Island Air are bringing them all in, and they would probably know better than anyone else.
You should contact the A.C. Chamber of Commerce, the San Pedro Business Associaton and/or check with the Belize Tourism Board. I am sure that you can find lots of info by google'ing statistics as well.
All I know is that there are at least 50 T-shirts on sale for every tourist passing through, so I must repeat my advice of not opening a T-shirt business, unless you have re-invented the T-shirt, in which case you'd probably not choose Belize as your launching platform.
Please, don't get me wrong. Lots of good people have come and made a good life for themselves (and others). Unfortunately just as many have come and lost everything, mostly because they didn't take the time to become familiar with the way of life and business here.
I don't know how much time you have spent in Belize; but a friendly advice would be to check it out for at least six months before you even consider risking your hard earned savings on a new business.
Not trying to disencourage you. Just being realistic, that's all...