Indefinite Pronouns, like collective pronouns, can be singular or plural. There are some rules to identify that. Like, "if it ends with -body, or -one, it is always singular".
Many can be both singular and plural. Several is plural. Some and All can be singular or plural.
The indefinite pronoun neither is always singular.
Neither is the negative counterpart of 'either' which is also singular. The basic principle of Subject-Verb Agreement says that the subject and verb must agree with each other, so you should use a singular verb because 'either' and 'neither' are considered singular; however, this principle is sometimes broken by a plural verb which can be used after 'either' and 'neither' informally.
