Imagine a gene with $n$ exons and $m$ introns. How many proteins are possible from that gene? Would all the proteins be isoforms?
I might be wrong, but aren't numbers $n$ and $m$ are connected as $n=m+1$?
Answer seems to be combinatorial: how many combinations of $n$ objects can be assembled under certain restrictions? Namely, how many isoforms certain gene can have.
Restrictions include: how many exon-intron junctions on codon (or precisely between codons), how many exons are actually contain protein-coding sequence of mRNA (some exons are coding untranslated region, 3'- or 5'-UTR, for example), how certain gene processes alternative splicing.
So, as you can see, answer will highly depend on sequence of given gene. As far as I know, maximum number of isoforms is limited by 5, even though there are genes with hundreds of exons.