The navigation I am currently wireframing for has close to seven sub menu items for each main menu and I was wondering if there is an optimal limit for the number of sub menu items
The right number to enable efficient navigation. Which isn't a lot of help, I know, but there is not a completely fixed answer. Sometimes, a large sublist will work, if the user can find what they need easily (if it is alphabetical, and there is a clear reason for many items). Sometimes, the lists need to be smaller, because the user needs to make decisions at each stage, and smaller lists tend to make for easier decision making.
So I would suggest looking at what the users are trying to do, rather than starting from an approach of menu size.
Traditionally, common wisdom used to hold that any list should always contain between five and nine items (seven plus or minus two), because humans find it hard to remember more than seven items at a time. However, that only really applies if the user actually has to remember all the items in the first place - and that only counts if the user can't make their choice without comparing the other options. For many situations, a long list of intuitively ordered items will trump an arbitrarily divided, needlessly deep tree structure.