Assume that all inputs are Taproot inputs (the cheapest input to spend) and since Lightning channel outputs are 43 bytes (Taproot won't change this) with each participant having an output channel, then the maximum channels you could open in one mega-transaction is around 9942 (just under the 1MvB limit, the transaction is 999181.5 vBytes). Such a transaction would be non-standard of course (current node rules don't relay transactions this large, but you can still mine them)

Now assume there is only 1 input. Then the maximum is around 23,000 outputs/channels (999435 vBytes)

All this is assuming all the transactions are aggregated though. Assuming each channel open was its own transaction then the limit is probably around a few thousand or so.

In the future I envision large Taproot conjoins like the first example into outputs that could be multisigs, singlesigs or Lightning channels (nobody can distinguish these with Taproot). Maybe someday we'll have cross-input signature aggregation as well, increasing this limit even higher. Hopefully we'll also have either hierarchical channels (google these, exciting stuff) or eltoo to move even more of Lightning off-chain