Thanks for the question, Tom. There's no one-size-fits-all answer to your questions, but here are some thoughts you may find helpful.
Let's start with how much of your overall portfolio you should have in real estate investment trusts (REITs). Many experts advise a specific percentage in real estate, such as keeping a portfolio that's 70% stocks, 20% bonds, and 10% REITs. But I look at things a bit differently.
Specifically, I'm a big fan of investing in what you understand well and what investments fit well with your goals. Not only do I have a firm understanding of how REITs work, but the long-term growth and income potential of REITs is a better fit for my goals than say, high-growth tech stocks. So I have more of my portfolio (roughly 30%) invested in REITs than many other investors do, because I'm comfortable with them and they're a good fit for my goals.
On the subject of how much should you invest in each individual REIT, there are a few things to consider. For one thing, I typically don't put more than 5% of my money into any stock, no matter how much I like it. If it performs well and ends up becoming a greater percentage of my portfolio, so be it. But I'm usually not willing to use more than 5% of my investable assets to purchase any given stock. This can be higher or lower for you, depending on your comfort level, but setting a maximum is often a smart idea.
Taking it a step further, I'd strongly advise limiting position sizes when it comes to riskier or more speculative REIT investments. As a personal example, I'm a big fan of retail REIT Simon Property Group (NYSE: SPG) and think there's lots of long-term value in the stock. But I also realize the next several years are likely to be a roller-coaster ride for mall operators (and their stock prices), so I only put about 1% of my portfolio into the stock. On the other hand, STORE Capital is more of a steady income and growth generator that should be a more stable investment, so I was comfortable investing my 5% max in it.
If it sounds like I'm telling you what I personally do instead of giving set-in-stone rules, that's kind of the point. While there are some principles to use in your REIT investing when deciding position sizes, it mostly boils down to your own risk tolerance and comfort level with REITs in general and the specific REITs you invest in.