Verified answer

Depends on what you are trying to say.

the = all of the "many customers"

these = the specific "many customers"

this = the number of "many customers"

The third sentence doesn't really make sense because it leaves a question unanswered (how many is this many?).

It depends but I would say and they brought this many customers as well

, and they brought many customers as well.

the last one i think "And they brought this many customers as well"

GOOD LUCK! :)