how many BTC-transactions are stored in 1 block (to my knowledge 1 block is 1MB)?
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You might get up to 4000 transations in a block.
To get the real maximum amount of transactions being possibly in one block:
Consider a block filled with the smallest possible (segwit-)transactions (1 input / 1 output)
The real crucial point is the sum of the non-witness part.
With 82 byte (non-witness part) transactions and a non-witness blocksize of 1MB we get a maximum of 12.195 transactions / block.
Compared to the corresponding non-segwit transactions (with size of 192 byte) this would be a maximum of 5.208 transaction / block.
Please note: This is an extreme example, since TX's mostly aren't only made of 1 input / 1 output. But it shows the limit of transactions (currently) possible.