There is no way to limit the number of entries a user can select on the parameter screen.
Does your report have any subreports? If not, then here's what you can try:
1. Create a main report that pulls a single record from any data - it doesn't matter what because you're not really going to use it.
2. In this main report, put the prompt that references the long list of values. I'm going to call this {?ListParam} for this example.
3. Create a new formula, that I'll call {@ParamCount}. It will be:
UBound({?ListParam})
This will give you the number of selected items from the parameter.
3. Make your existing report a subreport in the main report. In the Linking Expert, link from {?ListParam} in the main report to the same parameter in the subreport.
4. After you've placed the subreport, right-click on it and select "Format Subreport".
5. On the Common tab, click on the formula button to the right of the "Suppress" checkbox (DO NOT check the checkbox!). Enter something like the following in the formula:
{@ParamCount} > 20
This will suppress the processing of the report if more than 20 items are selected.
6. Create a new section at the same level as where you've put the subreport. In it put a text block that contains something like this:
This report is limited to 20 selected items from the parameter. You have selected {@ParamCount} items so the report will not run.
7. Go to the Section Expert for this new section and click on the formula button to the right of the "Suppress" checkbox. Enter the following:
{@ParamCount} <= 20
This will suppress the text if the user has selected 20 or fewer items.
-Dell