[Jill]

How many enantiomers are possible for aldoheptoses?

[AWK]

If you mean unbranched carbon chain number of pairs of enantiomers is 2⁴, number of all stereoisoners is 2⁵
For cyclic forms you should consider one more asymmetric carbon atom
and eventually take into account furanoses and pyranoses. This give you a total number up to 96 pairs of enantiomers (most of them are also called anomers)