I think there are around 3,500 broker-dealers in the U.S. right now. Most of them are registered in all 50 states. That number has been dropping for years though, and I might be off by some number of hundreds.
It’s really tricky to use the data you have to discern the number of broker-dealers. First, 1099s come not just from brokers but all types of custodians, which include banks and insurance companies. Furthermore, there are several types of broker-dealers in the U.S. that leave different paper trails. The big dividing line, for your purposes, is between “introducing brokers” and “clearing brokers.” Clearing brokers clear and settle their transactions and their clients’ transactions. Some clearing brokers (like Pershing and Apex) do this not only for their own customers but for customers who are introduced to the clearing broker by other broker-dealers (called, naturally, introducing brokers). The introducing brokers find customers, maintain the customer relationships, recommend transactions, and do lots of other things that are customer facing but, in general, they don’t handle customer funds and securities. (In the business, they are sometimes referred to as “$5,000 brokers,” because they need to maintain only $5,000 in net capital, unlike clearing brokers, who may have billions in net capital). The clearing brokers do all the unglamorous work of executing transactions, receiving and disbursing cash, arranging and extending margin, clearing and settling transactions, and sending out confirmations and statements.
The brokers you see 1099s from are clearing brokers, of which there are perhaps a few hundred. You most likely see lots of 1099s from clearing brokers who either handle lots of retail accounts (like National Financial Services, who is the clearing affiliate for Fidelity) or the five to ten firms like Apex and Pershing, which clear transactions for hundreds of introducing brokers.
There are also 14,150 SEC-registered investment advisers plus tens of thousands more who are registered only with the states. In general, they are going to be effecting transactions through some broker-dealer. The ones that deal with retail customers (and not just mutual funds, ETFs, hedge funds, or big corporations) will execute their transactions through a broker-dealer, usually through a platform offered by Fidelity, Vanguard, or Charles Schwab. Thus, if any of those investment advisers’ clients are also your clients, you will just see more 1099s from National Financial Services, Vanguard Brokerage Services, or Charles Schwab Clearing Services, respectively.
Long story short - there are thousands of firms being used by millions of retail investors but despite this diversity, most of them are going to generate 1099s from only a handful of firms.