How many base boards are recommended for a realistic simulation
Well, first of all, it depends upon the simulation. My first two routes, both in progress, are of complete shortline railroads ahd short lengths of their mainline connections. One has six boards, and one has three. The reasons for those numbers is that on the first, five weren't enough, and seven were too many, and on the second, four were too many, and two weren't enough.
Now, the way I decided that the one needed six, instead of seven or five, and the other three, instead of two or four, was that I approached the question like the producer of a play and first decided what kind of simulation I wanted to make, and next decided what I wanted to include by way of visual interest. Then I outlined the backstory (Every "real railroad" exists to satisfy a particular transportation purpose, and it's considerably easier to create a simulation that looks like a "real railroad" if one determines what the transportation purpose is before laying track.), which fleshed out some more details. When these decisions were made, the number of boards needed became instantly obvious.
Is it the more base boards the less performance.
While it is true that a route with two identical baseboards will impose more of a performance drag on the machine than a route with only a single baseboard, the answer to this question also depends upon a number of factors, and does not always hold true. Consider for a moment a route with two boards, one urban with a lot of buildings, tracks and other detail, and the other rural, with few buildings and little track. Depending upon exactly what's on the boards, it might well be that adding another board of urban content would put as much of a drag on the route as adding several boards like the existing one of rural content.
Since you're still trying to get to grips with Trainz, I'd suggest you spend time running all of the sessions of all of the routes that come built in. As you're doing this, I'd also suggest keeping a journal or log, in which you would note every aspect of each session and route that you like, and those that you don't.
Then, before you start trying to build routes, spend time experimenting with the various tools you'll be using, the landscape tools, &c. Study the various built in content in the game. This is important, because things are not always named in what seems to be a coherent manner, and the image you see in surveyor is not always representative of what something will look like installed on a route. When you know how to use the tools to achieve what you want, then is the time to think about building routes.
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