Is there a limit on number of sites in Craft CMS multi-site?

Yes, but with some caveats.

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Pri­or to Craft CMS ver­sion 5, there was no lim­it enforced on the num­ber of sites pos­si­ble in a Craft CMS multi-site. 

How­ev­er, as of Craft 5, the default lim­it is 100 sites, but you can bump that up using craft\services\Sites::$maxSites, but it is not rec­om­mend­ed to do so because of poten­tial data­base per­for­mance issues.