faSplit utility from Jim Kent at UCSC. Download and make executable (chmod a+x faSplit). Linux version linked, macOS available.
faSplit - Split an fa file into several files.
faSplit how input.fa count outRoot
where how is either 'about' 'byname' 'base' 'gap' 'sequence' or 'size'.
Files split by sequence will be broken at the nearest fa record boundary.
Files split by base will be broken at any base.
Files broken by size will be broken every count bases.
faSplit sequence estAll.fa 100 est
This will break up estAll.fa into 100 files
(numbered est001.fa est002.fa, ... est100.fa
Files will only be broken at fa record boundaries
and many other modes you can check by running faSplit.