I have a 24" x 12" x 15" tank which is filtered with an Interpet PF2 internal filter.

I have just added 5 platys which all seem to be doing pretty well. I'd like to add a few more (platys) but don't want to overstock the tank. I've found a few calculators on the internet but they seem to give inconsistent results.

Can anyone give me some guidance on the sort of stock level I can consider at this early stage?

Just checking - you did cycle the tank before adding fish didn't you?

If so, then I would suggest waiting a few weeks to see that everything is stable and then assuming you only want to stock Platys you could add up to 4 more making a total of 9.

They either need to be all one sex, or 3 males:6 females if you want both sexes. Be aware though that they are prolific breeders so you may end up with a lot of fry.

Yes the tank was fully cycled before adding fish and the water parameters are all currently stable.

Is 9 the absolute maximum or could I add a few more platys as the tank matures? If so, what would be a reasonable stocking level in say 6 months time?

Well your tank is about 14 gallons after taking off 10% for substrate etc so initial stock would be 14 inches of fish and maximum would be 28 inches when mature. Platys grow to 3 inches so 5 fish for now, and then 9 at the most. Bear in mind that as they grow, 9 fish in a tank that size will produce quite a biological load.

If you have both sexes you will have fry as well, though they may not survive in a small tank with 9 adults.