We use the Comodo certificate at work, there is no limit on the amount of code that you can sign. You buy a certificate for a year, 2 years, etc. And you can sign as much as you want during this period of time. In addition, if you correctly mark the time stamp, the signed code does not expire when the certificate expires, so as soon as you sign it (with a valid time stamp), it will be signed until you have to recompile.
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