I think your 'During' should be 'While' and 'add' should be 'ad'.
I'm not really sure if it's the right word to use, but I would use the hyphenated 'passers-by'.
Are you talking about the possibility or the past event?
Hi. Have I used the correct tense in this example: "During reading this text 152 passersby could see your add."
'During' is wrong. 'While' is better, but "While reading this text, 152 passersby could see your ad" means that the 152 passersby could see your ad while they were reading this text."
How about, "While I was reading this text, 152 passersby could have seen your ad", meaning "in the time I took to read this text ...."
"could have seen", "might have seen" are correct.
Also note that ad not add is the abbreviaton for advertisement.
I am not talking about a past event. I have established an advertising company and I want to advertise it. What I want to say is that while (in the time) a customer is reading my ad (an advertisement about the services my company offers), certain number of passersby could hypothetically see his banner only if had already made use of my offer.
"In the time it takes you to read this text, 152 passers-by could have seen your ad."
It's conditional. "They could have seen your ad, if you used my services." It's only past in the sense that they've lost time by not subscribing to your service. The message is that they don't have to continue to lose potential customers.
"In the time it takes you to read this text, 152 passers-by could see your ad." This is present tense, but I don't think it's an improvement.
"In the time it takes you to read this text, 152 passers-by will be able to see your ad." This is future. I don't think it has the motivating force of the past tense. How do you know what will happen in the future? The customer is more likely to believe that you know the past. You know that, with your service, 152 people could have seen the ad, and didn't. The customer can work out the odds for the future.
In summary, I think the past tense/conditional version is the best.
Yes, you are- the person has just read the advert, you are talking about reading it- the fact that the advert is still there doesn't change that. Like Raymott, I prefer the could have read form- the person you're targeting has just read your ad, and their potential customers have missed the chance because you're not displaying your target's ad.