Dear Sir,
No you can't buy a under construction flat before selling your flat. Under construction flat can only be bought within three years from the date of sale of old house.
But ready flat can be purchase before 1 year from the date of sale of old house or within 2 years from the date of sale of house.
No there is no cap in holding flats under section 54. But there is cap in 54F.
So you can use 54
Please feel free to call/revert in case of any doubts
Thanks and Regards
Abhishek Dugar
CA CS B.Com