How many known time/result combinations does it take to guess a HOTP/TOTP secret?

Have a look how the HOTP (TOTP is just a special case based on a time for now) is calculated. It is using HMAC based on hash function either SHA1, SHA2 (or MD5 in worst security case) of secret seed and some counter. It is returning some part of the result as a PIN.

As an attacked intercepting the PINs, you are trying to find out a secret seed of unknown length.

Based on the definition of hash function, it not reversible (unless it is broken). You can not simply reverse the operation even if you would have the whole hash. But you have 6 to 8 decimal numbers. You also do not know where they fit into longer decimal string -- this is also changing based on last byte).

So what are the options you have? The easiest way would probably be to brute-force all the possible values for seed (for example 32 B hexadecimal string) and compare them with the intercepted data. This could work with the TOTP since you can know also the time when it was generated (30s time frames). But in the HOTP, you have no idea what the counter is (unless you would manage to intercept all the PINs.

I would say, the strength of the OTP is its complexity of the algorithm and simplicity of use. I am not giving a proof why it is hard, but I hope the answer summed your question and answers why you can not simply do that.