Segwit gives you 1.7-2MB blocks. People telling you otherwise are lying.
and immediately gives us ~2MB blocks (whether that's a good idea is up for debate, but if that's what you want, you get it).
IF 100% of people move funds to segwit p2wpkh/p2wsh keys..if only 50% of people do it. then expect 1.3mb-1.5mb capacity increase.
oh and its a 1 time max possibiility.. you cant resegwit a segwit to get upto 3mb..
oh and once people start using different features like CSV CLTV (adding extra bytes) confidential payments(adding upto1kb) even if its the same 2in 2out tx...
that will add more data (mb weight) to a block.. but no increase in tx count..
EG we could see
1.0mb=2500tx today
2.1mb=4500tx after activation and
IF 100% utility of keys being segwit.. and then
4.0mb=4500tx when using the new features