Alkaline has no internal limit to the amount of concurrent search queries. However, if the server is very busy, Alkaline will keep requests pending and will service them as soon as resources are available. Scaling tests have shown that Alkaline handles several dozen requests per second at ease, but this number strongly depends on the data search and the sizes of databases.
We advise stressing Alkaline with at least twice the load you expect on your server for testing purposes. You can, for example, use Microsoft Homer for this purpose, available freely at http://homer.rte.microsoft.com .
Several of our clients production servers successfully handle an average of 3-4 requests a second. We have tested up to 25 requests a second under stress conditions.