In Numbers^x, Promotions^x, Verbs^x

1. There HAS been an increasing number of promotions. 2. There HAVE been an increasing number of promotions. I believe that "an increasing number" is singular, making correct .

Can anybody give me an answer to this question? Which is correct? There HAS been an increasing number of promotions. There HAVE been an increasing number of promotions. I know that "number" can be plural or singular and differentiated by "a" or "the" before the subject, but the words "an...