I have just finished my wireframe, and am about to contact designers to get some mockups done. My wireframe is about 20 pages in total, yet only 5 or 6 of the pages are significantly different from each other design-wise. Do I need to get all 20 pages designed (every little detail) or do I just have the designer do a few, and then give the developer a style guide so they can put the rest together?
Believe me--both designer and developer will appreciate it if you can minimize the differences between the pages as much as possible. It also makes sense from an overall design point of view as well, as even different pages/sections of a website need to fit with an overall look in order to present a cohesive appearance.
It's hard to say for sure without seeing your wireframes to get an idea of what you want, but to actually answer your question: I'd say turn over all of the wireframes to the designer, and then give them their creative freedom. Try and group them together by what you think are similarly styled pages, and then rely on them to return 5 or 6 templates, versus 20 separately styled pages.
A good designer will be able to set things up to minimize differences and also minimize work for the developer. A good developer in turn should be able to work with the basic templates in order to achieve the differences you want between pages when he's actually implementing them.
Depending on the technology underlying your website, it will be more or less straightforward to develop a set of templates from your wireframes. Each template represents a different section of a website and may share some elements with other templates while having some of its own unique styles and layout properties. In my experience of creating WordPress-based websites, an 'entry level' collection of templates might include:
With, for example, an added level of customisation by including multiple layouts for use with Posts or Pages, such as a selection of landing page templates. As has been suggested above, pass your wireframes to a web design and development team and work with them to find ways to consolidate the number of templates.
It may be helpful to think of your website as a collection of cookie cutters, rather than as a collection of pages. Once you've designed and manufactured the cookie cutter you can create as many cookies in that shape as you like! In other words, a template is like a cookie cutter, once you agree a standard shape for each section of your site, you can create endless numbers of pages in that standard shape. Equally, you may want one sort of shape for your home page, a different one for regularly published articles and another one for pages describing products or services. We've been working through a similar issue with a client which inspired a couple blog posts on website vocabulary and concepts which might be useful to you http://www.sproutee.com/wordpress-vocabulary-101/ and http://www.sproutee.com/website-vocabulary-101/
What I typically see is that designers design the different elements (headers, content, forms, buttons, ...) of the pages. In other words, they might be delivering only 6 pages, but a good layouter will be able to piece together the different (20-some, in your case) pages based on that.
This more generic approach allows you to quickly expand the number of pages without having to have them designed each time.