I just came across the same issue. Both biblatex and csl have “number” and “issue” fields, but their meanings seem to be quite different. Do I understand correctly that biblatex’s “number” (string) should always be mapped to CSL’s “issue” (number) and biblatex’s “issue” (string) should also be converted to CSL’s “issue” whereas there is no biblatex equivalent of CSL’s “number” (number)?
Can CSL deal with the issue being a string like “Summer” or should one always make sure it is converted to a number?
The biblatex manual says this about the difference between the two fields in biblatex:
2.3.11 Journal Numbers and Issues
The words ‘number’ and ‘issue’ are often used synonymously by journals to refer
to the subdvision of a volume. The fact that biblatex’s data model has fields
of both names can sometimes lead to confusion about which field should be used.
First and foremost the word that the journal uses for the subdivsion of a volume
should be of minor importance, what matters is the role in the data model. As a rule
of thumb number is the right field in most circumstances. In the standard styles
number modifies volume, whereas issue modifies the date (year) of the entry.
Numeric identifiers and short designators that are not necessarily (entirely) numeric
such as ‘A’, ‘S1’, ‘C2’, ‘Suppl. 3’, ‘4es’ would go into the number field, because
they usually modify the volume. The output of—especially longer—non-numeric
input for number should be checked since it could potentially look odd with some
styles. The field issue can be used for designations such as ‘Spring’, ‘Winter’ or
‘Michaelmas term’ if that is commonly used to refer to the journal.