Yes, you can start a blog and keep it private until you want to publish it publicly. You can change privacy settings, or you can just keep your posts as drafts instead of publishing them.
I think when you are saying pages you probably mean posts.
Posts are individual entries. There can be many posts on a page, or just one. You can control that in your blog settings.

It is rare to make any sort of real money with a blog.
If you set yours up on blogspot, which is a common free one (and where mine lives), you can add Google ads.
When people click on the google ads from your blog, you get a fraction of a cent. Over time, these can add up. But Google won't send you a check until you accumulate $100.

If your blog gets more than 1k hits per day, you might start getting approached by ad sales people who want to market on your blog.
There are also scammers who pretend to be ad sales people but who really want to rope you into some scheme. If your blog is not getting 1k hits per day, people who approach you are probably not legit. (This is from personal experience getting approached by shady characters or businesses that seem like pyramid schemes, ymmv.)

Thank you! I'm not concerned with the money aspect (it was mentioned to me) as much as wanting to share the letters with others who enjoy that sort of thing. As she wrote sometimes as many as three letters a day, I was wondering if there was a limit on the posting, lol! Now I just have to interpret his handwriting - it's a bit sloppier.

Sounds like a cool project and would be the basis of a great blog.

It's not a given that if you put these on a blog, you'd have the copyright. Take a close look at this similar (and not at all uncommon) situation.

It's not a "don't do this" stipulation, but if a descendant discovered your use of the letters you might get a cease-and-desist letter. Nothing big about that, but know what you're doing. The post recommends you do your due diligence to find descendants before publishing, and that makes sense to me.

With most free blog engines, you can build up a bunch of posts, save them as drafts, and publish when you're ready. It's a good idea to have at least 5 or 6 solid posts before you start marketing your blog to people.

Do it for love, not money. There are a lot of blogs like this, so it's not especially likely to go viral enough to be financially productive.

Who's going to care about your blog and why? Think about your audience so you can tailor it to them. It's fine to do a blog for a miniscule audience. But don't expect the world to take notice unless there's some significant hook to the story, or you're a relentless promoter and skilled media writer. Most blogs have a teeny audience. You could always post on MeFi projects to get a few eyes on it early and get some recommendations. My friends that have blogs are diligent about posting on FB and Twitter when they have a new post up, which is good, because unless you let people know there's new content, no one will really find it, as people just don't troll blogs via bookmarks much any more.

It sounds cool. Had a similar experience in high school, pre-internet, discovering a cache of letters from a couple in the 1930s traveling with their jazz orchestra. Good luck!

Thanks Miko!

Another comment to say don't do it for the money, unless there is a clear cut, assured path to the money you are comfortable with. Cream rises to the top. That being said:

Do you have a good camera/scanner?

Just with what you wrote, it makes me think of a personal find and Griffin & Sabine.
You could use any of the autoblog free services or very easily customize a clean, simple site that makes it easy to transition into hard copy publication.

First, work with what you've got, privately. Or you could spit ball the process publicly. Do your due diligence and a well crafted letter could get you funded for your effort.

Even if it is fun, remember it is time and effort, which so oddly people forget are limited resources. Make it worth it to yourself.

Now, think fun fun fun! Nothing like the beginning of a fresh new project!