Also, if my budget is 7000 usd total. How many s19 miners can i afford? (Or L7) Not many. I find that I can make much more profit with a few dozen L3+ than with one or two newest most energy efficient machines. For example 100 units of L3+ will give me about 4000 usd profit a month. While 2-3 s19 will only give me like 1200 usd i think ?
This is for my opinion and it's just my opinion, Maybe what you say is right that you can make money there with a few dozen L3+ miners compared to 1 or 2 S19 miners. From what I know, L3+ is cheaper compared to S19 miner, although it is weaker and older, unlike S19 miner which is better and newer, that's why it is more expensive than L3+ miners.
Now, regarding your question that you have 7000$, my answer to you is that with this amount that you have, my rough estimation here is that you can buy from 7-10 L3+ miners and I estimate the hashrate that you can get here is between 400--510TH/s. And the chances you can earn here are probably playing more or less 1000$.
If you buy S19 miners, my rough estimation that you can get hashrate is more or less 110TH/s which with the difficulty and value of bitcoin it is possible that you can earn around 2000$ I guess . So if you're looking for good miners I suggest that I think it's better to buy 1-3 of the S19 miners.
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https://www.asicminervalue.com/miners/bitmain/antminer-s19-95th https://simplemining.net/asic-miner-profitability/antminer-s19