One share is the minimum. Most mortgage REITs trade between about 3 and 25 dollars a share, and plenty of brokerages allow fractional shares, so the practical entry is a few dollars. US stock commissions at the large retail brokerages are commonly zero now, though you should check your own broker's schedule rather than assume.
The two levels your friend meant. Level one is anything you pay to buy: commission if your broker charges one, plus the bid to ask spread, which on thinly traded small mREITs can be a cent or two per share and matters more than it sounds if you trade often. If you buy an ETF that holds a basket of these instead of picking one, that fund charges an expense ratio, historically in the range of roughly 0.4% to 0.5% a year for the main mortgage REIT funds. Confirm the current figure in the fund's prospectus, because they change.
Level two is inside the company. Many mortgage REITs are externally managed, meaning an outside manager runs the portfolio for a fee, often around 1.5% of shareholders' equity per year plus in some cases an incentive fee above a hurdle. You never see that as a line on your statement. It comes out before the earnings that fund your dividend. The exact terms sit in the management agreement, which is an exhibit to the annual filing, and it's worth reading who the manager is and whether the fee is on equity or on assets.
No license and no insurance are needed to own shares. You're buying a listed security like any other.
