Over the past 7 days, there have been around 99419 hits on the "too-many-titles-for-query" deprecation warning from user agents starting with "WikipediaApp", for example "WikipediaApp/2.4.184-r-2016-12-14 (Android 6.0.1; Phone) Google Play". For those with access to Kibana, see this dashboard.
For most users, a maximum of 50 titles may be specified for the titles parameter. Titles in excess of 50 are ignored with a warning, although someday I'd like to make that an error. If you have more than 50 titles to query, you should query them in batches. This limit applies to other multi-valued parameters as well.
The queries being made seem to mostly resemble action=query&format=json&titles=...&continue=&prop=pageimages%7Cpageterms&piprop=thumbnail&pithumbsize=320&pilimit=...
: Users with the apihighlimits right have a limit of 500 rather than 50.