Let's assume Agassi was born in the early to mid 80s.Say, 83-85.Against whom? He's not as good as Nadal. That leaves one opportunity, in 2009, and I think Fed takes that because Fed is a better claycourter.
If we're assuming that He's exactly 10 years younger, then sure. That leaves 2004. He couldn't beat Guga then so he's unlikely to beat Guga now. That leaves the 2004 FO, the 2003 FO and the 2002 FO. So let's be generous then and say that he gets 3. Maybe in 2015 vs Stanimal.
Australian Open, who does he beat? Stan in 2014? Safin in 2005? I'll be generous and give him the 2000 AO, the 2001 AO, the 2002 AO, the 2003 AO, the 2005 AO and the 2014 AO.
As for Wimbledon who does he beat? Hewitt in 2002? Goran in 2001? I don't see either. Zero wimbledons.
US Open. 2009, 2014, 2003. I don't see him getting 2009. 2003 and 2014, maybe.
So looking at it, that gives us:
2002, 2003, 2004 FO.
2000, 2001, 2002, 2003
He'd win his last slam at the FO at 2004. And all of these are dependent on being born in 1980 like Safin.
If he were born in 1985, then I don't see him winning more than a couple of slams.
Like 04, 05(Nadal wasn't developed on clay yet), 09, and depending on his age, 15-16 as well.
As for AO and USO, conditions were much faster back then and still he managed to win 6 slams on HC, in these slow conditions he would fare much better. You are also forgetting he was Federer's toughest competition at 34-35 in 04-05 USO.
As for Wimbledon, yes this was his weakest surface I agree. But he still managed to win one , and make another final in the 90s era, of fast grass, serve and volley, and grass specialists. Sampras at his peak was a much better grass player than Federer. So Agassi would perform better in this slow grass era. Baseliners, like Nadal, Murray,Djokovic, have won 9 slams on grass. Surely Agassi can win quite a few Wimbledons.
A prime/peak Agassi if motivated, in this era could be a double digit slam winner.
Now let's analyse the slams in each year one by one. First with Wimbledon.
I'll start from 2002, as Agassi would probably in the juniors or not developed yet prior to that.
If Leyton Hewitt was able to win Wimbledon in 2002, a younger Agassi would have definitely won a Wimbledon that year. Agassi is a much better player, a better baseliner and has better weapons than Hewitt. A 2002 W would have been Agassi's. 2003 Wimbledon against Federer, Agassi would have had a chance, his opponent that year was Phillippoussis. 2004-2008 would have probably gone to Federer, Nadal, or Roddick etc but Agassi would have fared a chance and would still be in contention for them. 2009 W Agassi would beat Federer , Federer was in horrible form throughout the tournament, and Roddick played better in the final even though his peak years were long gone, and he didn't even deserve to be in the final in the first place, Nadal withdrew, Djokovic not yet developed on grass. Agassi would have a strong chance to beat that Fed in W.
Then that leaves 2013, 16, 18 as well. Agassi could have beat djokovic in 13 and 18(tired from 5 setter better chance than Anderson) , and obviously Raonic in 16 as well.
So that leaves 6 Wimbledons. And frankly it's not too far fetched when you have a guys like Djokovic, Nadal,Murray have 9 W despite being defensive baseliners.
Now the French Open.
2003 FO would have been his, as well as 2004 FO.
He would have had a chance in 05 FO, as Nadal was not yet developed on clay, and his opponent was Puerta, and he almost took him to 5 sets.
Then 09 FO, Soderling was mentally and physcially done after beating Nadal. Federer's form wasn't even that good in the tournament. Struggling against Haas and Delpo, 5 sets etc . Agassi would have had his chances.
Then there's 15-16 FO(Agassi would be early 30s), a declined Nadal , Agassi would have won those years as well. Agassi is a better clay courter than Wawrinka. Djokovic's form in 16 wasn't that good . In 15 he was good, but was taken to 5 sets by Murray. Agassi could pull out a win vs Djokovic.
2013 Agassi would have at least made the final and performed better than david Ferrer against Nadal.
Now Coming to the Ao and USO.
2003, Agassi beats Roddick at USO.
Even 34-35 year old Agassi pushed peak Federer to 5 and 4 sets in 04, 05 respectively.
An Agassi in his prime would win 04,05 as well.
05 AO, Safin would have lost to Agassi. Hewitt is not Agassi.
Then say he loses out on slams in 06,07, motivational issues, personal reasons,i injuries etc..
He still has 08 AO, Agassi could beat Tsonga and under developed Djokovic.
US 08, Agassi would have beat Mono and out of form Fed.
In 09 AO , Nadal was taken to 5 sets by Verdasco despite Verdasco choking in crucial moments. Agassi with his ROS and backhand , would have been a much bigger threat.
US 09, considering he would've won A0,FO,and W that year, he would be going for the CYGS, in front of an american crowd. His only competition was Delpo, and Fed, Nadal out of form. Federer as we know becomes mentally affected when he loses so many matches to a rival. Agassi would have the upper hand against him. 2012 US would have gone to Agassi instead of Murray, same with 14 AO and USO, and 16 USO.
In 2017, which is the weakest year in tennis, he would be in contention for slams seeing Fedal vultured 4 slams when there was no competition that year.
So that leaves us with around 6 W,6 FO, 8 USO, at least 4 AO.
So that's around 25 slams. That's my hypothesis.