It depends which tank they are in. As Helen says, you'd have more options in the big tank. Honey gouramis are the least aggressive gouramis but they still have the potential to be nasty.
In the 90 litre I would risk only one more gourami, a female. If you have 2 males, adding a female might make them fight but you can remove one in that case. If you have 2 females adding a third won't cause problems. If you have a pair, adding a second female should be OK too - though in this last case keep an eye on the original female for any signs of picking on the new one. The only time I've had problems with honey gouramis is with 1m2f and the females fighting.
In the big tank, it depends if you have something to break up line of sight - tall plants, a tall bit of decor etc. They need to be tall as males hold territories on the surface. With this proviso, you could up the numbers to half a dozen. But get more females than males.
Honey gouramis come in three colours; natural, yellow and red. The natural colour is the fish in the profile on here. They are not common in shops as the female is silvery grey and who wants grey fish
They are the easiest to sex as they are different colours, though males can look washed out in the stress of a shop tank. The yellows are next easiest as stressed females show the stripe along their sides. With this colour, choose the ones with stripes for females and the ones with no stripe and usually a bit more orange towards the tail.
The easiest way is to take someone with you. Choose the fish you want then send your companion to get someone to serve you while you keep your eyes on your fish. When the net goes in the tank, even the males will look washed out and it will be harder to see the difference.