Mecho rules:-

General Rules

All submitted pages must contain a link back to MechoDDL.
Link directly to your download page, not your main page.
No popup adverts are allowed.
Do NOT submit any Adult content or Demos.
Do NOT change the page after your downloads are added.
Do NOT add fake files to your main page.
Downloads will be deleted from the list if they are missing, or the page has been changed after submission.
You may submit up to 20 downloads every 24 hours.
NO mention about quantity of buttons/links.

Nexusddl:

Rules
No metalinks from unrelated content
No frame breaking
No full page ads
No misleading links or advertisements
Links must go to the download page, not the homepage
Again NO mention of limit.

twilight is the only one that does mention a limit.