Can I log-in to my wallet on new PC using just my wallet number and passphrase? I was into bitcoin years back. I just stumbled upon my wallet number and passphrase. Can I use these info and sort of log-in using any client I can download online?
It depends what you mean, but the answer is probably "yes" either way.

Traditional wallets use a wallet.dat file, often encrypted with a password.  In this case, you would need the file and the password in order to use your wallet on a new computer.

Hierarchical deterministic wallets (HD wallets) use a single seed (possibly what you are calling the "wallet number") to regenerate all the addresses in a wallet, thus recreating the wallet.  Often times this seed is stored as word seed (possibly what you are calling the "passphrase"), meaning several randomly generated words that can be fed into an algorithm to regenerate the wallet seed, which in turn regenerates the whole wallet.  If this is what you mean, then you could restore your wallet with either just the seed, or the passphrase.  How to accomplish this from a UI will be determined by the particular wallet you are using.