You need ArcGIS 10.1 to publish a service in ArcGIS Online.
Can you divide your shapefile of 50000 features into 50 shapefiles? No you can't. Since you asked, here's why...
There are two reasons why you can't add more than 1000 features to a web map. One is a performance issue and the other a technical limitation of client applications, like web browsers and phones.
When adding one or more shapefiles to the web map, all the features in the shapefiles get stored in the web map. Adding a lot of data to a web map can make it large. The practical limits to web map size are 5-10 MB, but this also depends on capabilities of the client application viewing the web map as well. Web maps get downloaded to the client, so a large one can take a while to download, especially on mobile devices. So you don't really want your web maps to get too big because people will then think your map is slow.
The shapefile features stored in the web map actually get drawn as graphic elements by the client. There are limits to a client's ability to draw these graphics. In the web browser (most notably older verisons of Internet Explorer) ArcGIS.com displays an error message when the map doesn't draw completely because you've reached the limit of the browser's capability to draw graphics. This is why we recommend generalizing your data before adding it to a web map. Every vertex of every polygon gets stored and thus has to be drawn by the client. There can be a lot of unnecessary vertices depending on the scale you're viewing your data. Excess vertices just cause the client to reach its drawing limits sooner. So in reality, the 1000 feature limit is an general limit, not a hard limit enforced by ArcGIS.com.
Sometimes people mistakenly think the limit is 1000 features per shapfile and proceed to add 3 shapefiles, each with 1000 features. This is incorrect because it's actually the client's ability to draw the features, independent of how many shapefile layers there are. The tricky part is that different clients have different limits. A web map may display fine in Google Chrome and Firefox, but not in older versions of Internet Explorer.
The solution, as I wrote in a previous post is to create a web service of your data--either a map or feature service (optionally a hosted service on ArcGIS Online if you don't have ArcGIS Server). However, I will point out that the same client drawing limit applies to feature services (both hosted and ArcGIS Server), and this is where some confusion may arise. While a feature service itself is not limited in the number of features it can store, the features are still drawn by the client and subject to the same drawing limitation as when the features are stored in the web map. That's why you need to be very thoughtful about setting scale dependencies on your feature services. For example, you don't want to view a parcel database when zoomed out at the world level. It's just too many features to request from the server (there are limits on the size of queries the server will accept too) and too many features for the client to draw.
Thus, a feature service may not the best solution, especially for displaying large amounts of data on the client. If you need this ability, you should probably be using a map service, and for maximum performance a tiled map service. However if you need to support editing, then you have to use feature services. But, you need to configure the properly to perform well.
Thanks,
Mike
