People seem to have become millionaires in previous bull cycles with modest investments in BTC & ETH. If I invested in 1 BTC, is it enough to make me a millionaire by the next bull run, or are there an elite number of alt coins that could help me safely achieve what BTC & ETH once did?
Sadly the short answer is No.
The long answer is that if you are willing to grind buy Bitcoin even if the price is 25k at present and you will likely make a lot of money on the next bull run. While altcoins are rarely going to be anything good maybe in short term you will make profit but there is not guarantee on that and it is also proven to be more risky than other methods of investment.
Also few people actually become millionaires. This is an exaggeration, from what I have seen over the last few years.