We've created 4 or 5 search term targeted landing pages on our site with good success. Within a week or three the pages are ranking top 5 for decent long tail terms that will bring us 5-25 sales per month.
These pages are regular product category pages, look like any other page on our site and are linked with a real (albeit bottom of the page) link from a legitimate page. The meta info, H1 and intro text on the page is different. The product mix is very similar, if not the same as other pages on the site.
We want to create more of these pages - maybe 25 or 30 total in order to grab all of the top long tail terms. As long as these are "real" pages, and are linked from within our site, should we be worried about overdoing it? I'm not sure how G would differentiate any of these new pages from the other 40 existing "real" pages like them - the only difference being how frequently they are linked to throughout our site.
Every page on your site is a (potential) landing page, with a few exceptions (like terms and conditions pages etc.) all have a prospect of attracting relevant traffic.
It sounds to me like you're viewing "content" and "landing pages" as somewhat different entities, which could be a shirt-sighted approach.
Nonetheless, as every page you create is a potential entry page from organic search, you can create as many as you like ;)
If you're creating a series of very similar pages, then if the current batch all operate independently, then you're likely to be able to do so with further pages without risk of triggering duplication filters.
If you think the pages would fail a manual review, then you may be wise to reconsider the approach, and consider how you might target an increasing variety of keywords in a way which cannot create a negative impression.