In the construction There is an X, the entirety of X is a noun phrase (NP) - in your example, *an even number of rules. If you're specifically focused on the (singular) element number* within that NP, you can get away with using the singular verb form *is* in your exact context, but this would be far less likely with, for example, An even number of people were* invited to the party.*
– FumbleFingersOct 02 '17 at 14:07
The subject is "there". The verb tense is wrong -- it should be plural "are" to match the noun "rules" (cf. "There are a number of rules to be observed"). But with existential "there", and reduced "is", many speakers ignore the rule and always use the singular: "There's an even number of rules".
– BillJOct 02 '17 at 17:55
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– Oct 02 '17 at 21:52
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@EdwinAshworth The problem is though that you cannot helpfully redirect readers to a question that will necessarily help them, if it only addresses half the question. And the linked to question is closed anyway. [There's also the fact that, as FF pointed out, this question is not quite the same as the linked-to one in terms of agreement, because as he says, is will potentially work here.]
– Araucaria - HimOct 02 '17 at 22:19