Google App Email and Gmail both restrict filters to a maximum of 20 per account. You can't add more filters to an account, but you can use a workaround to use more filters.
Google App Email
1) Login to whichever account has Administrative priveleges.
2) Click the link to "Mange this domain".
3) Click on the "Organization & users" tab.
4) Add a new email address. Skip to step #5 below.
1) Create a new Gmail account. Skip to step #5 below.
5) Login to your account that already has 20 filters in use.
6) Click on "Settings", or the small gear that contains "Mail Settings" if on a newer account.
7) Select "Forwarding and POP/IMAP".
8) Add your new email address your just created to be a forwarding account of all email.
9) Confirm the forwarding by logging in and clicking the link in the email sent to your new email address.
10) Click on "Settings", or the small gear that contains "Mail Settings", within your new account.
11) Select the "Filters" tab.
12) Begin adding more filters.
13) Repeat the process if you need even more filters.
NOTE: You can have two different browsers open if you don't want to login and out of both of your email accounts.