This depends on which package you use to provide your ftp server program. With some programs you can specify a default umask on the daemon's command line. With vsftpd you can specify it in /etc/vsftpd.conf. Here's a snippet from the default config file:
# Default umask for local users is 077. You may wish to change this to 022,
# if your users expect that (022 is used by most other ftpd's)
#local_umask=022
I think clients can also change their umask from the default from within an ftp shell.